Hard to tell from the pictures but it appears to lichen -
Lichen - Wikipedia, the free encyclopedia
".....You are more likely to find lichens on unhealthy trees, but lichens are not the cause of the trees' deterioration. Lichens are related to mold and moss and grow well on trees suffering from other issues, because the plants are not healthy enough to fight them off. Lichens do little damage other than blocking some sunlight from getting to the tree, but you can remove them for aesthetic reasons."