Are you certain that what you're seeing is moss? Moss can grow on shaded, moist parts of trees (not just the north side, as advised by the old adage about finding your way out of the forest). Moss usually develops in patches and can easily be scraped off the trunk or stem. The greenish patches you are seeing may instead be lichens. Lichens are a combination of a fungus and a green or blue-green algae. They form a crustlike growth that is difficult to scrape off the trees, rocks, and other hard surfaces on which they develop. They, too, prefer shady, damp locations, though some grow in full sun. Lichens may be green, blue-green, orange, brown, or yellow. They don't penetrate the surface of the tree or cause harm to it. Therefore, no control is needed.