It could be nothing, but it could also indicate that the humidity is not high enough or that he's having trouble getting things moving, digestion-wise. Is he still in the bowl? What is the humidity? What is your substrate? When was the last time you fed him? What do you feed him?

There's ways to fix those things, but hopefully he's just playing a cruel trick on you, as frogs sometimes do. I once lifted my water bowl to find one of my White's upside down and sporting a horrid color and thought he was dead, only to find out later he liked laying upside down under the water dish.