Discoloration of cactus: Fungi or Mite?

I’ve been caring for this cactus for about 3 years now, and it seems happy based on the amount of growth.
Recently (about 2 months ago) I started to notice a discoloration/change to the outside of the cactus. I can’t tell if it’s from spider mites, or some kind of fungus. It’s in the same pot as another cactus that doesn’t have the same issues. Can someone help me identify what exactly is going on? Pictures attached…