It is a plant. I read somewhere that you need to use a weed spray. It is a moss. Your roof is too damp and shaded.

solution of bleach hot water, some scrubing and rinse rinse rinse
A power washer may do the trick to - but if not done right will leave roof streaky

Are there trees overhanging the roof that may be the culprit

Moss B Ware, at homedepot ?

Here is a link that might be useful: http://www.winroof.com/moss.html

No scrubbing or powerwash! You don't want to remove the agregate (those small grainy particles) or your roof will be shot. Bleach and water should kill it. May take more than one application. And remember to protect whatever may be growing under downspout from runoff. Check out other home repair forums on GW or talk to roofing people. Hope its not too late. Sandy

Alkaline part of bleach, supposedly isn't a good thing to put on asphalt type roofing or concrete tile roofs as it ruins the 'tar' paper. I've read that somewhere - I don't have the hands on experience. I do have some extra expensive fancy new roofing... maybe I could experiment.

On "Ask This Old House" Ron Silva installed
Zinc strips underneath the roof shingles to
prevent mold from regrowing.
He first sprayed the roof with diluted bleach & water (don't forget to cover plants & hedges).