How much moss is on the roof. I would first suggest leaving it alone. Anything you do will dislodge granules and the granules are the protector of the asphalt of the shingle.
Now if you must try some kind of cleaner, go to your local ABC Roofing supply store and ask for Shingle Shield. Best thing I've found to clean a roof. Apply with a garden sprayer and rinse with a garden hose. DO NOT, I repeat DO NOT use any form of pressure washer.