The green is not mold, but algae or lichen that is able to find enough moisture to live on the siding. It creates its own food from photosynthesis. You can turn the lights out on this by using a zinc spray, or by hanging a zinc strip where rain will wash residues of zinc down on the slime. Needless to say, zinc is toxic to the slime, but not to you, pets, or most plantings. Can buy products at Home Depot and most garden centers.