What is moss?
Moss is a bryophyte. A "simple" plant, NOT!. Not really simple, it's quite an amazing plant that you could study your whole life and still not understand everything about it.
Moss needs sunlight, growing medium and water to grow. It can grow in full sun, provided it gets enough water. Moss uses chlorophyll to make energy from the sunlight.
Moss can go dormant when it is too dry or too hot. During the summer, people may think that the moss has died. But then when the fall rains come, the moss swells up and seems to "come out of nowhere". People claim they didn't have it last year. The spores are small. The tendrils of first moss growth are small. But it grows steadily in size when the weather allows. It is a fascinating plant. I believe moss is the only plant that can dry completely and not die.
There are at least 20,000 species of moss. Different kinds grow on different parts and climates of roofs. Some types of moss will fall of the roof easily when poked when it's dry. Other types of moss will cling resolutely, wet or dry, to the granules and sooner rip off the granules than be removed from the roof. This type of moss must be killed for sure before trying to remove it from the roof. There are about 12 species of plants that grow on roofs in the Pacific Northwest.
Moss can grown in full sun on tile roofs provided it gets enough occasional moisture. These clumps of moss grow larger and larger, trapping dirt that is running off the roof. The more water, dirt and sun it gets, the faster it grows. Moss grows at a slower rate on South facing roofs because the roofs dry out quicker.