Looks like moisture damage. Doesn't have to be from setting the phone directly onto something wet either. This happened with my 5S leather case over time.
My guess is that Apple will exchange simply because their customer service and aim at satisfaction is tremendous. But at the same time, you should prepare yourself for this type of a case taking on different looks. Leather is a funny thing, especially when on a case this thin. The colors can change as well as the texture. Mine is showing wear that the one on my 5S never did.
Still my favorite case by far however. The feel in the hand, in my opinion, is unmatched. By the way, welcome to iMore Sam