deciduous trees that are losing their leaves
We have a group of deciduous trees that have this mold growing on the bark and now the leaves are falling off. We live on Long Island, NY.
I'm not sure the type of tree in the image.
Lichens can be harmless to trees but the fact that you have leaf drop may or may not indicate this is part of the issue.
I would look for insects or disease on the leaves. Also clean up any leaf drop from the tree and dispose of.
When you indicate a group of trees are experiencing the same symptoms that is the biggest indication of a problem.
I do think it would be a good idea to have the trees inspected by a professional arborist.