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 are solely fungi, also an independent kingdom from a botanist’s perspective. They get energy by feeding on organic matter where they live. They don’t use photosynthesis, so they don’t need sunlight.
Source: https://www.washingtonpost.com/lifestyle/home/whats-that-green-stuff-growing-on-the-outside-of-the-house/2019/02/15/82419e4c-2bef-11e9-b011-d8500644dc98_story.html