Staining on an asphalt roof

If the roof is well shaded by the trees, it could be mold, mildew, or algae, or a combination of the three. Increasing sunshine a bit and air circulation would result from some tree pruning or removal, making for a more healthy roof. Big box stores like Home Depot sell zinc compounds, such as Moss Out, that will attack the stain. It is diluted with water, then applied with hose-end sprayer or pump sprayer. Make sure that whatever you get, is not going to damage the shingles.