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I believe that's an internal worm making it's way out of the body. I would suggest you place the fish into a bare hospital tank so that when the worm does leave, it has no place to hide ( in the substrate) to reproduce and infect your other fish. You also want to keep watch for any secondary infections to the wound the worm will leave exposed. Treating for any infection should be done in a bare tank anyway so why not get a head start on the process. Once the fish has recovered, I would also suggest a course of treatment for internal parasites ( ie. API's General Cure or Tetra's Parasite Guard or their ingredients ) before returning the fish to your display tank. This will help insure that there are no other parasites living inside your fish.