Tang has leprosy?

[Gizmo]

I'm not sure what is wrong with it. I've had it about a month, but he was always hiding in the rockwork so I didn't notice this. I've since changed up the rockwork to something more open, so he's not always hiding, but I would like to get him well again. Any ideas? Sorry about the pic quality, my camera is at work and had to use the cell.