Those dark, unsightly streaks are a form of algae known as Gloecapsa Magma, and unfortunately, roofs provide the perfect environment for it. Gloecapsa Magma can grow wherever there is darkness, humidity, and a hard surface to attach its spores onto. Besides being an eyesore, this algae can weaken your shingles if left untreated for too long. If you see Gloecapsa Magma on your roof, the best way to get rid of it is by professional
roof washing.