What’s that green stuff growing on the outside of the house?

A: The tiny organisms that grow on the roofs and walls of houses can be moss, algae, lichen, or mildew or mold (two terms often used interchangeably). All can create greenish blotches on siding. Especially from a distance, they can look alike, especially to a casual observer.

They are strikingly different, however.

Only mosses are classified as plants. Like other plants, they use photosynthesis to turn sunlight and water into the food they need. Algae once considered plants, are now in a kingdom of their own, but they also depend on photosynthesis. So do lichens, which are combination organisms that have algae or bacteria living within a structure made of fungus, with the fungus dependent on the food the algae or bacteria make via photosynthesis. Mildew is solely fungi, also an independent kingdom from a botanist’s perspective. They get energy by feeding on the organic matter where they live. They don’t use photosynthesis, so they don’t need sunlight.

If there is no texture, you’re probably dealing with algae or mildew. It’s harder to distinguish between these. A green color usually points to algae, but not necessarily. Some algae are red, and although mildew is often black, they can also be green, yellow or brown. Because algae depend on photosynthesis, you’re more likely to find this type of growth on surfaces exposed to the sun — such as your house. Mildew thrives in the dark, which is why indoor surfaces are far more likely to harbor mildew than algae.

All four kinds are most likely to become established on damp because they all need moisture to thrive. The abundance of rain this past year could help explain why you are seeing this growth for the first time. Also, if it’s been decades since you painted, there’s a good chance the paint is more porous than it was initially, keeping the surface moist longer and giving organisms an easier place to take hold. Differences in the porosity of the brick or previous paint layers might explain why the growth is only in the new section of your house. There could also be a difference in how quickly the wall sections dry after a rainstorm because of differences in insulation or other construction features.

Whatever the type of growth, you may be able to simply scrub it away, especially if it’s algae or mildew. Start with the gentlest of procedures, using mildly soapy water and the type of soft-bristle brush or sponge you might use to wash a car. If that’s not enough, buy a siding cleaner labeled as effective against the type of growth you have. A mildewcide won’t work against algae, and an algicide won’t work against mildew. Lichens can be particularly difficult to remove without using a suitable cleaner.