One of my guys had a pink belly after a shed, made me kind of nervous. But it went away after a couple of days. So if you have a thermostat on your UTH and are pretty sure the temps are accurate, just give it a few days before panicking.
BUT, if you don't have a thermostat on your UTH, and he recently moved the substrate and was sitting on the glass, it's probably a burn :/. But fortunately it doesn't look too bad, yet anyway---sometimes burns look a lot worse the next day. So, if it doesn't get any worse, just unplug the UTH until you can get a thermostat, and put him on paper towels until his next shed. If it does get worse, best to call the vet.
ETA: oops, saw that you do have a thermostat. What kind? What's it set at? Where's the probe? How confident are you that the temps are accurate? If the glass temp is under 95 it won't cause a burn.
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