Hmm.... That's really weird. You normally don't see any sort of injury like that from f/t prey. Mouse claws are not very strong even when they've got a live mouse controlling them, and the way snakes eat, the feet are usually pinned back in a way that the claws couldn't do damage.

So, what I'd do is use a wet Q-tip to swab away and loose 'goopy' stuff in the area (be gentle so you don't break off any scab tissue and reopen anything). Then use another Q-tip to apply a solution of diluted chlorhexidine (brand name Hibiclens, you can find it at most pharmacies) 1 part chlorhexidine to 4 parts water. Do that for a couple days. If after a day or two you don't see improvement, if swelling increases, or if the color changes (so instead of pinkish tissue you start to see black/brown/purple tissue) I'd say it's time for a vet.
At the moment it doesn't look infected, but you don't want a mouth infection to start up and get out of control, so if it doesn't respond to that quickly I think you'll want vet intervention before it really becomes a problem.