You can easily scrape off the existing mold and afterward apply a standard cleaner to remove any excess. There are products on the market that can prevent mold from ever returning. Some are thin strips of zinc that slide into place under the shingles. You should install these shingle shields at or near the top of the roof, extending its entire length. It is only necessary to install one complete strip-not several layers. What the shingle shields do is release a microscopic amount of zinc every time it rains. It will run down and cover all the shingles, protecting them from mold. The amount of zinc released will prevent any return of the mold, but it is not powerful enough to do any damage to the shingles themselves.