Is my tree dying?
Your deciduous trees are fine. The gray-green patches you see on the trunks are a kind of harmless lichen (sounds like "like-in".) Lichen is a combination of fungus and algae, which only uses the tree as a structure. It carries on photosynthesis on its own, so doesn't suck any nutrients out of the tree at all. Very common and normal.
Here's more about it: https://extension.umd.edu/hgic/expert-plant-picks-lichen