No worries it is Lichen.
You can look on line to see how to kill it but I have it in all verities on my trees and it doesn't seem to hurt them.
It is also an indicator of clean air.
Really slow growing trees have more of it because the bark doesn't shed it off fast.
https://www.google.com/url?sa=t&rct=...PH8ALm8y9UauKA