I notice from time to time with my tree frogs they can darken in patches and I think its due to temp. It doesn't seem to effect them so Im hoping thats all this is. The other thought was a burn mark from a light? If your frog is eating well then to me thats a good sign. I would definitely keep an eye on the mark though and if the frog becomes lethargic or stops eating then best to get to the vet asap.