It's probably an ulcer. What are your water parameters? If your ammonia is more than 0, nitrates more than 40 ppm, and nitrites more than 0... you need to do a water change (20%). Have you added anything like melafix? If not, I would add some.
Also, have you done the research to make sure you aren't adding too much salt, and not too fast? The fish will need time to adjust to the salinity... there are two different ways for a fish to deal with either freshwater or salt water.
-----
Phishie

"An ounce of prevention is worth a pound of cure."