1) Remove existing mold. A pressure washer on a conservative setting
works well. You should be able to remove mold without damaging
your shingles. (Yes, I have experience with this).
You can also use bleach to remove existing mold, but I prefer the
pressure washer. Test a small area first to determine that the bleach
treatment is ok. If you've got some spare shingles in your garage or
shed, test on one of them.
2) Purchase copper strips designed for the task of preventing future mold.
The strips aren't that expensive and they work well. Copper salts of any
sort are pretty deadly for all plant life.