My instant reaction would be mites, about to shed or the tank is too hot. Does he ever go into the hide on the hot spot? If he never goes into it, there is a good chance the tank is too hot or the humidity is too low. It generally isn't horrible if they soak lot, but it can mss up their skin to constantly soak. Just keep an eye out for it. Put another hide in the tank on the cool side and see if the stops soaking and uses that hide instead.