Algae growth on roofs is typically visible as dark staining with light-colored streaks below flashing where runoff carrying disolved zinc from galvanized metal flashing has inhibited growth. Discoloration from algae growth is most common in the southern US and in coastal environments, but may be found across anywhere adequate moisture for growth is present.
Algae discoloration is difficult to remove from roofing surfaces, but may be lightened by applying a solution of chlorine bleach, trisodium phosphate, and water. Solutions for these ingredients may vary between shingle manufacturers and it's best not to make direct recommendations but to recommend a qualified contractor.
High pressure washing systems should not be used to remove algae!
Lichen will not damage shingles, but efforts to remove it are likely to remove granules. It's best left in place.