How to tell mold from mildew on a Naples home - what grows on stucco, pool cages and tile roofs in Southwest Florida, how to test it, and when it stops being a cleaning job.
The difference comes down to depth. Mildew is a flat surface fungus - grey, white or blackish, slightly powdery - and it sits on top of stucco, screen mesh or a lanai slab without changing the material underneath. Mold grows into a porous or organic material, looks fuzzy or slimy rather than flat, and points at a moisture source that has to be fixed rather than washed. On a Naples home, almost everything you can see from the driveway is mildew or algae, and it comes off with a low-pressure chemical wash. The growth that genuinely needs a remediation contractor is nearly always somewhere you cannot see from outside.
Up north, exterior growth is a summer problem. Here it is a year-round one with a hard peak. From roughly June through October, Naples runs afternoon storms most days followed by heat, which means exterior surfaces get wet daily and rarely dry fully overnight. Add salt-laden air off the Gulf, irrigation running before dawn, and dense landscaping that blocks morning sun, and you have close to ideal growing conditions for four different organisms at once - three of which are harmless to the structure and one of which is not.
The reason to care about which is which is that they do not respond to the same treatment, and two of them are a sign of a problem that washing will not solve.
Mildew is the most common thing on a Naples exterior. On stucco it reads as a dull grey or blackish haze that makes a wall look tired rather than obviously dirty - homeowners often think the paint has faded when the wall is actually just covered. On screen enclosure frames it collects in the corners and along the bottom track. On white vinyl trim and gutters it shows as speckling.
The identifying features are that it is flat - it has no depth or fuzz when you look at it close - and that it is on top of the surface. Wipe a patch with a damp cloth and mildew lifts, leaving stucco or paint that looks the same as the clean area beside it. That test alone settles most cases.
Mold looks different in person. It has texture - fuzzy, cottony or slimy - and it comes in colors beyond grey and black: olive green, orange, and the dark green-black that people associate with the word. Crucially, when you wipe mold off a porous surface, the stain stays, because the organism has grown into the material rather than sitting on it.
On the outside of a Naples house, true mold is uncommon on stucco itself, which is mineral and not much of a food source. Where it does turn up outdoors is on organic material that stays wet: wood fascia and soffit, a screened lanai's wooden furniture, mulch beds pushed against the wall, and the underside of a deck. Indoors it is a different conversation entirely, covered further down.
The dark streaks running down a tile or shingle roof are the single most misidentified thing in this market. They are usually Gloeocapsa magma, a hardy blue-green algae that feeds partly on the limestone filler in roofing material and spreads by airborne spores. It is not mildew, it is not mold, and scrubbing it is both ineffective and a good way to damage a roof.
It matters because the treatment is specific: a roof needs a no-pressure chemical application designed for the substrate, not a pressure washer. On a tile roof high pressure drives water under the tiles and can break the ones you walk on; on shingles it strips granules and takes years off the roof. If you want the numbers side of that decision, see what roof cleaning costs in Naples.
The green haze on screen mesh, cage frames and the shaded half of a lanai slab is green algae, not mildew. Screen enclosures are a Southwest Florida special case: the cage holds humidity in, cuts airflow, and shades the deck for most of the day, so a caged pool deck frequently grows faster than the open driveway twenty feet away. Salt film off the Gulf gives it something to hold onto, which is why coastal cages in Naples and on Marco Island foul faster than inland ones. That specific job has its own walkthrough in removing salt film and algae from a pool cage and lanai.
You do not need a lab. Three checks in order:
This is the Naples-specific case. A house closed up from May to November sits through the entire wet season with the air conditioning set high or off, no one rinsing the lanai, and landscaping growing in around it. Owners come back in season to a pool cage that has gone green, a north wall that has gone grey, and pavers with a slick film on them.
Two practical consequences. First, growth that has had five uninterrupted months is materially harder to remove than the same growth caught at two months, and lichen in particular may have established. Second, and more important: a closed-up house is exactly the scenario where interior mold becomes a real risk, because an air conditioner that is off or set too high does not pull humidity out of the air. If you return to a musty smell inside, that is not an exterior washing question - deal with the inside first. There is a broader seasonal checklist in getting a seasonal Naples home ready after the off-season.
Naples exteriors are mostly stucco, and stucco behaves differently from the vinyl siding common further north. It is porous and alkaline, it holds moisture in its texture, and it is easy to damage with pressure - a tight tip held close will erode the finish coat and leave permanent wand marks that are far more visible than the mildew was, which is why cleaning stucco safely comes down to solution and dwell time rather than raw force. Painted stucco adds a second constraint, since aggressive pressure will take the paint with the growth.
Pavers are the opposite problem: they clean up well, but sand comes out of the joints and sealed pavers can go cloudy if treated with the wrong chemical. Screen mesh will not take pressure at all - it stretches and tears - so cages are a soft-wash-only surface. And the wood in a lanai is the one place on the property where you may be dealing with real mold rather than mildew.
For everything on this list except true mold, the method is the same and it is chemical rather than mechanical: a sodium hypochlorite solution with a surfactant so it clings, applied at low pressure, given ten to fifteen minutes of dwell time, then rinsed. The chemistry kills the organism; the rinse removes the dead material. That is what a soft wash is, and it is why it works on surfaces you cannot safely blast. The comparison is laid out in soft washing vs pressure washing.
The one thing worth adding for this climate: rinse the landscaping before and after. Cleaner running off a two-story wall lands in the beds below, and Naples plantings are close to the house almost everywhere.
A pressure washer with no chemical knocks the visible growth off the surface without killing it, and on porous stucco it does something worse - it opens the texture up slightly, giving the next generation more to hold onto. That is why the wall that was blasted clean in March can look worse than the neighbor's by September. It is also why "it came back in two months" is usually a method problem rather than bad luck.
The related question of what pressure does to a hard surface is covered in does pressure washing damage concrete.
Be straightforward about the limits. Call someone other than a pressure washing company when:
None of that is an argument against washing the house. It is an argument for not treating a wash as a fix for a moisture problem.
Most Naples homes land on an annual exterior wash, with two realistic exceptions. Homes close to the Gulf, homes under heavy tree cover, and seasonal homes that sit closed through the wet season often need it more like every eight to ten months to stay ahead of it. Homes in full sun with good airflow and no irrigation overspray on the walls can sometimes stretch past a year. Timing it right after the wet season ends, rather than right before it starts, is what buys the most visible months. The specifics are in how often you should pressure wash a home in Naples.
Is the mildew on my stucco a health risk? Exterior surface mildew on a masonry wall is not generally an indoor air quality concern. Growth inside the home is a different matter and should be assessed on its own.
Will bleach from the store do it myself? On a small shaded patch, diluted household bleach with a little dish soap and a soft brush works. It fails on large areas because you cannot keep the surface wet long enough for the dwell, and it fails on roofs and second stories because you cannot get to them safely.
Does vinegar work instead? Not well on exteriors. It has some effect on light surface mildew indoors, but it does not have the killing power for established algae on stucco or a roof, and it will not touch lichen.
Why does the north side always look worse? It gets the least direct sun, so it stays damp longest after the daily rain. North and east walls, and anything shaded by a screen cage or a live oak, foul first almost every time.
My roof streaks came back a year later. Was it done wrong? Not necessarily. Roof algae is airborne and re-seeds from neighboring roofs, so a treated roof does get recolonized eventually. Coming back in a few months points at a method problem; coming back after a couple of years is normal for this climate.
Can pressure washing spread mold? Pressure alone can aerosolize and scatter growth without killing it, which is one more argument for a chemical-first approach that kills in place before anything is rinsed.
If you would rather have it identified and handled in one visit, see what house washing in Naples covers, or start with pressure washing in Naples.
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