Hi Jen:

Sorry, I missed you post and didn't respond earlier. I would remove the frog from the colony into an isolation tank (bare tank). It is probably a bacterial infection or injury. I would try a dose of ampicillin (sold in tropical fish stores as "Fishcillin"). Follow the label instructions. It should clear in one or two applications. Let me know how it goes.

As an alternative, if you are really good at handing Xenopus, you can apply 2 drops of Bactine on the wound twice daily until clear. Bactine, NOT Betadine! Bactine has been used in clearing up minor wounds in most amphibians.