Confidence: I'm as sure as I can be.
Notes: Powdery (leprose) thallus, in this case with quite a well defined margin. Oftentimes these Lepraria species cannot be identified unless you have a chemical laboratory at your disposal, and this may well be one of those occasions. I have been scanning the photo at highest magnification to see if there is any sign of brown hyphae beneath the granules, but too much of the orange alga, Trentepohlia, getting in the way. So I won't speculate, beyond pointing out that L. sylvicola is said to be widespread on oak in the west and I cannot see anything that would rule it out, but ID has to be just to genus.