They are lichens on the tree you posted. They pose no real threat to the tree itself and produce their own food from air, moisture and light. The tree is not actually a host, just where they hangout
Lichens need ample sun for photosynthesis to carry out. Here in MD I usually find them on the trunk on lower scaffolding branches where there is more light. On healthy trees with a full dense canopy, you probably will probably find none in the crown.
I have heard folks hitting them with a copper based fungicide if they become a aesthetic issue. The only thing I can see as negative is they make an excellent overwintering ground for pests. I would monitor them harder next season.