The best thing to do is to clean the roof with 10% bleach to 90% water solution. Rinse it with a pressure washer on a low setting. Cleaning the roof on a sunny day is best; the sun actually helps the bleach to work more thoroughly.
After you get the mold off, you can install a copper or nickel piece of flashing along the peak of your roof. Every time it rains, the rain will cause some of the flashing to wash down along your roof, effectively self-cleaning the shingles and fighting mold spores before they have a chance to make themselves at home.