Not sure what that is I would definitely keep an eye on it if it were me. I'm not a vet, but I don't think it is mouth rot that usually starts in the mouth on the gum area. However that doesn't mean it isn't some other form of fungal issue. If it gets worse might want to consider a good reptile vet. Good luck sorry I couldn't have been more helpful.
I would take a wet Q tip and see if it rubs off. It could be residue from a feeder, but you don't want it sticking to the lip for a long time anyway. If it doesn't rub off, I would inspect the gum inside carefully for any sign of a cut or sore. Look for a reddened area and any swelling. I don't see swelling anywhere.