Is this algae or moss? How to remove it without damaging the shingles?
It's algae now &amp; will become moss later &amp; it will come back until you properly pitch the gutters &amp;/or clean the gutters. Any wetness that continues long after everything else has dried, like you have, indicates a drainage problem. However, a tablespoon of bleach to a cup of water with a gentle &amp; light brushing (don't scrub the shingle granules off the shingles) will remove it. Bad gutter installer, as indicated by lower gutter.