Does the fish have any other symptoms? Is it still eating well.
How are your water readings?.
If your readings are good, the fish is behaving normally, and everything else in the tank is fine, I would think this might be lymphocystis. Check out the diseased fish sticky at the top of the forum for a picture of this disease. This disease does not require treatment.
If your water readings are off (or if other fish are also ill), then I would favor a bacterial infection. A nice water change may help. If not, the fish may have to be isolated and treated with antibiotics.
More information would be helpful.