It's probably some sort of infection. Can you tell me exactly what you are currently keeping it in (size of enclosure, substrate, temperature, lighting, humidity, plant types...) and if you've made any changes to it's set-up recently? Also, is it the only animal in the enclosure, and where is the terrarium stored? When's the last time it ate, and how much of what did it eat? Are you giving it vitamin/calcium supplementation?
Sorry for all the questions, but I need a lot of answers to properly answer your crucial question. Some pictures would be helpful, and if you have a herp (amphibian and reptile) vet near you, I suggest you pay them a visit with your buddy. I hope all will be well!