Bleach, 50/50 with water, spray it on when it's cloudy out so it doesn't dry out to fast. Wait a day or two then rinse, repeat any spots you may have missed. Once your done have a roofer apply zinc straps across your whole roof line one course down from the ridge and you'll never have the problem again. Very inexpensive solution to your problem. Roofing manufacturer's used to mix zinc in with the granules but stopped for some reason, never had that problem years ago, only with wood shingles.