We had a tree trimmed that was in front of the house. The branches reached out over the porch roof keeping it in shade throughout most of the summer. Over the past few years an algae-like growth began to grow on the shingles under the shade of the overhanging branches. We called a place that does roof repair in Brooklyn NY to come out and take a look. They explained on the phone that the growth would not likely compromise the protective properties of the shingles but would just be unsightly.
Cleaning it is problematic. It can stain most shingle colors and look more prominent on some and less on others. Even after cleaning there are usually telltale signs that something was there. We thought that the bright summer sunshine would bleach the area clean. I imagine that would take so much time that the rest of the roof would never match the formerly shaded area. I had the roof repair crew clean the area and chemically treat it to kill the growth and halt it from coming back.
They did a really good job cleaning the shingles that had the growth. You can barely tell there was-whatever that algae-like stuff was-anything there. We plan on replacing both the upper roof and the porch roof as well as the one on the garage in a few years. The shingles are still in really great shape, so we cannot justify the expense just to fix a slight discoloration due to growth on shingles caused by overhanging tree branches. It is our own fault. Keeping the tree trimmed in the first place would have made it so we never had this expense. We learned the facts about tree maintenance the hard way. Some people have it worse. An improperly maintained tree may have fallen on their house.