Answer: The green growth is probably lichen and usually is not harmful to trees but it can hold too much moisture against the bark which can cause rot. Of more concern are the dead and bare branches. While I can't tell for sure without actually inspecting the tree, I suspect a root problem - perhaps water stress. If the tree is valuable you might consider hiring an arborist to assess the health of the tree and determine whether or not it can be saved. Wish I had more encouraging news for you.
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