Nice tree. Reminds me of what you can see in Brazoria county, Texas. I don't think there's much you're going to be able to do to effect any change. I can't see the moss, but it may be Tillandsias. T. recurvata known as 'Ball Moss' and T. usneoides known as 'Spanish Moss'. Those are epiphytic plants and will not damage or harm the tree. Most things you see on the surface of the limbs and trunk are lichens and moss and they are not parasitic plants and are harmless. Parasitic plants such a Mistletoe do feed of the living tree. My guess is the tree has endured lack of rainfall in it's recent past. The effects of severe drought can show up in a tree as long as five years after it has ended. So it depends a lot on the tree. It's not dead yet, so there's hope. Looking at the way it grows makes me think that it had growth trimmed out of it or that at one time there was a lot of understory growing up in the branches - other trees, vines that choked out growth or large shrubs. Likely those caused deliberate trimming of the canopy at some point. If there are no obvious areas of the trunk and large limbs showing disease, it may just need a few good seasons to grow more leaf canopy. You might call your county agent and ask them about it. I hope it recovers. It's a great speciman.
