Why Black Streaks, Moss and Fungus Grow on Roofs When algae and fungus get together, they form something new, lichen. Lichen is a photosynthetic fungus that has roots that wrap around the granules, feeding on the nutrients of the GM and the filler in the shingles. Once on the roof, lichen cannot easily be removed. Even if it dries out, it can come back to life when rain or moisture appears. Scrubbing or power washing the lichen will only do more damage.