I recommend a soft, bristle brush and a mixture of 1 cup Bleach per 1 gallon of water. (You can go heavier w/the Bleach if the mold/mildew is 'heavy'.) Don't power wash or scrub too hard. Repeated passings of the brush over the affected area will slowly loosen the 'problem'. Heavy pressure by any means will loosen the asphalt granuals, thus, dramatically shortening the life of your shingles.
Start from the lower 'edge' of the roof and work your way up. DON'T start from the top down. (You'll create 'streaks' if you start from the top!) As you work your way up, wet down the previously washed part of the roof. That way, the dirt from above doesn't 'dry' on the clean areas below. DO be mindful of the vegetation below. The Bleach can kill! If your downspouts empty into a garden or the lawn, go to the Home Center and see if there's a biodegradeable cleaner. (Maybe someone on the Forum has a more specific suggestion on this note.)