If you just got the fish and your LFS just got it in, it might just be an air bubble. We had this happen to a fish and we flipped out thinking it was popeye and that we were going to lose the fish. Luckily, it went away with time.
On the other hand if it is popeye, you might want to quarantine the fish anyway. IIRC there really is no cure for it if it is the disease.
