Below is a partial excerpt cut and pasted below from ARMA:
Algae discolorations to remove from roofing surfaces, applying a solution of chlorine bleach, trisodium phosphate, and water. Solutions for these ingredients depend on the amount of discoloration. First, gently disperse this solution on the roofing surface. Avoid scrubbing the surface, as this friction may loosen and remove granules. Caution! High pressure washing systems for algae removal should not be used."
Owens Corning Roofing manufacturers says the same thing:
"USE CHLORINE and TSP" "DO NOT USE A PRESSURE WASHER"