I have these, too. They appear to be completely harmless. I seemed to get a population explosion of them when I first set up my terrarium, but it gradually stabilized, and now I only see them if I dig something up or move something. They seem to prefer a certain piece of driftwood that is constantly moist and growing a bit of mold on it. I wouldn't worry about them. I was told that they're probably harmless, and who knows? Maybe animals would even eat them! I'd only get concerned if you notice them crawling all over your frogs and if the frogs seem to not like it and are unable to get rid of them.

They are mites. They look for places with high humidity and food. I get them on occaission in my tarantula enclosures. Fortunately, I have the option of letting everything dry out and the population drops/disappears. You can purchase predatory mites(Phytoseiulus persimilis[good]/Hypaspis miles[better]) that will clean them up if it becomes a major problem.