what is the environmentally safe way to remove & prevent moss from growing on the roof?
We use a bare galvanized wire fastened to the ridge of the roof. 

If you’ve ever noticed a “clean” area under (downside of slope) chimney flashing or metal roof vents, you’ll understand the use of galvanized metal to keep the roof “clean”.  (I’m sure some chemist could explain why the galvanized coating keeps the asphalt shingles “moss-free”. )