Hi and welcome to GoY, a photo would be the best way of us identifying the problem. It is possibly lichen but without an image difficult to say for sure. Is this a Japanese acer or one of the larger trees?

Welcome to GoY.

I think this is likely to be lichen too.
if it is then it shows you have nice clean air where you are. It wont harm the plant. if you dont like it then you could 'pick it off'. if its a big tree dont fall off the ladder though :o)

welcome to GoY from me too.