It's hard to say from the photo. It could be some sort of polypore, but there are a lot of things that match that description. A local extension service or arborist might be able to ID it for you.
The photo seems to show a hollow trunk. Old trees can get all sorts of opportunistic things munching on them. I'd recommend that you get an arborist to take a good look at the tree. If it is really old be prepared to hear that nothing can be done. Trees don't live forever.
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Looks like a lichen of some sort to me. Nothing to worry about. However, that hollow trunk is another more troublesome issue and it worries me too. I agree you should have an arborist take a look at it. It is sad but trees really do not live forever.