Mold, Mildew or Algae on Your House? How to Tell

Quick ways to tell them apart

Algae is usually green, sometimes yellow-green, and forms a flat film or powdery coating. It likes shaded walls that stay damp, which is why north-facing walls and areas behind shrubs turn green first.

Mildew shows as gray, black or brown speckles or patches, flat to the surface. It is common under eaves, around windows and on shaded stucco and siding. A simple test is a drop of diluted household bleach on a small spot. If it lightens within a minute or two, it is likely mildew or algae rather than dirt.

Mold is a broader term, and mildew is a type of mold. On exterior walls, what people call mold usually looks thicker, fuzzier or raised, and can be black, green or white. Mold growing in large patches, or appearing on the inside of walls or in attics, is a different situation from exterior surface mildew.

Where each tends to grow

All three need moisture, shade and something to feed on, like dust or organic film on the surface. Near the coast, the marine layer keeps walls damp into the morning, so growth is more common there. In the Valley, growth tends to stay where water hits: next to sprinklers, under leaky gutters, behind dense planting and on walls that never see direct sun.

Our guide on green algae on north-facing walls goes deeper into why one side of the house always seems to turn first.

How the cleaning works

Exterior algae, mildew and surface mold all respond to the same approach: a soft wash. A diluted cleaning solution goes on with low pressure, sits for a few minutes to kill the growth and loosen it, then gets rinsed from the top down. The solution does the work. Pressure alone tends to smear growth around, leave roots behind, and damage stucco or siding.

Plants are wetted before and rinsed after, and wash water is kept out of the storm drain. Los Angeles Municipal Code section 64.70.02 prohibits discharging untreated wash water from paved-surface washing and mobile operations into the storm drain system unless a permit allows it.

What to expect after cleaning

Algae and mildew usually come off cleanly and the wall looks even again. Very old growth on porous stucco may leave a faint shadow that fades over the next couple of weeks as the dead material weathers off. On painted surfaces, deep staining sometimes only lightens.

Growth tends to come back over time in the same spots, because the conditions that caused it are still there. A shaded wall near sprinklers may need cleaning every year or two, while a sunny wall may stay clean much longer.

A light maintenance wash before the rainy season, roughly November to March, can keep growth from getting a head start during the damp months.

When growth signals a moisture problem

Some growth is just shade. Some is a symptom. Pay attention if it returns very quickly after cleaning, appears in a sharp line or a single concentrated patch, shows up near a roof edge, window or pipe, or comes with cracked or soft stucco, bubbling paint or a musty smell inside.

Common causes include an overflowing gutter at one corner, a sprinkler head soaking the wall daily, a leaking hose bib, failed window caulk, or soil piled above the weep screed at the base of stucco. Fixing those is worth more than any wash. If growth appears inside the home, or there is water damage in the wall itself, that is a job for a mold or building professional, not a pressure washing company.

Getting it handled

Small patches on a single-story wall can be handled by a careful homeowner with a garden sprayer, a diluted cleaning solution, a soft brush and a hose. Whole-house growth, second stories and roofs are where professional soft wash equipment saves time and keeps everyone on the ground.

When Say Wash It cleans growth off a house, we point out likely moisture sources we notice during the final walkthrough. House washing runs $0.35 per square foot of exterior wall as of October 2026, and you can get a starting number from the instant quote on this page.