It is probably lichens. They are totally harmless. In New Zealand they try to get as much lichen growth as they can. They even have yard furniture that is totally covered in lichens.

Most anything that kills lichens kills just about everything. There is no need to do anything, as lichen is not detrimental to the health of your plants. But if you do wish to get rid of it  if it's covering ornamental bark, for example  there are a couple of very simple things you can do. First, you can get a scrubbing brush and water and gently rub off the lichen. Or you could cover the trunk or limb for a while with shade cloth; eventually the lichen will die from a lack of sunshine.

Moss grows on the ground and is also highly prized. It is the ultimate ground cover for rhododendrons and azaleas. Again, most anything that kills moss kills everything.

Here is a link that might be useful: Moss and Lichen