If you were to search for [link removed by moderator] you may find my company, pressurePros, Inc. We clean roofs professionally. Some interesting answers here. those streaks are a cyanobacteria called Gloeocapsa Magma. They cannot be pressure washed away. A treatment with a potent fungicide is the only good course of action. No more pressure than what comes from a garden hose should be used to rinse a roof.