The most important thing to remember is that over growth of plants etc that shade the roof oftentimes is the real culprit here. The roof needs sun to dry out. Overgrown tree branches shade the roof allowing this moss to develop. Cut the tree branches back and get some sun on the roof. Then follow KMS link and have someone install copper or zink strips along the top edges of the roof. This should help prevent the re-occurrence of the moss.
...»
The roof however will need to be properly cleaned free of the moss first. There are companies that do just that type of work. Just remember that if your roof is getting up there in age, you may end up needing a new roof as well.
There are products available which contain zinc. In fact, the newer asphalt shingles have zinc in them to help eliminate mold. Once clean, you can get zinc strips and put them on your roof. There may be professional roof cleaners in your area. We used one for $250 and $90 annual touch up. It was well worth it.
http://www.networx.com/article/roofing-rx-tr...
The roof however will need to be properly cleaned free of the moss first. There are companies that do just that type of work. Just remember that if your roof is getting up there in age, you may end up needing a new roof as well.