Anything else living with the snapper? Any jagged rocks? If not, it could be an infection. Keep the water very, very clean. and be sure to provide basking spots. it should clear up on its own.

I guess it is possible that it is chasing its own tail. Make sure it has plenty of food. Supplying them with live fish not only provides sustainance, but enrichment. And stay away from those gross, bright orange feeders that they have at Petsmart and the like. Very unhealthy, diseased things piped full of dyes so they look pretty.