I agree with the other respondent. It could be a lot of things, but the most likely culprit is a cage that never dries out, causing a fungal infection. It's fine to have a water bowl in the cage, but it shouldn't be so big as to dominate the floor space of the cage. The baby may then be constantly wet. The substrate should not be waterlogged or constantly damp at the surface, and the glass of the cage should be free of water droplets.
If the problem isn't severe, it's possible to dry out the cage, disinfect it, and then wait for the next shed. Sometimes this problem will heal on its own, if it's not severe. But if it's severe and it sounds like it is, a vet should be called.