That looks to me like little spots where the lichens have been killed, whether by friction of some sort, toxic exposure (unlikely), or a pathogen to the lichens itself.
Now this is pure speculation on my part, but when you zoom in real close, the light spots are always on the margins of a healthy stand of moss/algae/lichens. The lighter colored areas (where circular) have a darker center, and the overall pattern looks like your lichens have caught a fatal disease that is slowly spreading across the tree.
I don't think you need be concerned at all. If you are really interested in a solution, I think you would need to contact a university that has a department that specializes in Lichenology, or some related graduate level studies in plant pathology. I tried to find some research on viruses , fungi, or bacteria that are pathogenic to lichens, but that's a very deep hole to take a swim in, and I don't have enough expertise to find stuff that technical.
As far as that goes, we don't even know if the green growth on your tree is moss, lichens, or algae. A really good closeup picture might get us closer, but I think I've seen several different flavors of green "stuff" growing there, and this isn't exactly the area of our expertise.
I tried making sense of this article, but quite frankly, this article about pathogens in lichens is completely beyond my working vocabulary. See for yourself: https://www.frontiersin.org/articles/10.3389/fmicb.2020.561344/full
Needless to say, it didn't have any matching pictures, either.