Can't answer for sure, but it could be an intestinal blockage of some sort.
What have you been feeding? What has it been eating? Does your tank have a bare bottom or substrate? What kind of substrate? What other organisms are in your tank? How long have you had this fish?
There are, as you can see, many unanswered questions. If you want to give us a good picture of your system, there are a few more questions to answer:
How old is your tank? When did it originally cycle?
What kind of system do you have (tank volume, dimensions, bio-filtration method, the equipment you use, any carbon or other chemical filtration, kind of substrate, etc.).
List all specimens & sizes in the tank (fish, inverts, corals, clams, snails, crabs, shrimp, etc.).
If you have a refugium and/or sump, what is living in it?
Do you use a quarantine tank and procedure?
Feeding schedules.
If the fish was recently acquired (4 weeks), two more questions: Do you know what sea/ocean it came from? How did you acclimate it ââ"šÂ¬Ã¢â‚¬Å“ what procedure?
Do you use any vitamins? Fat additives? Any elemental or other additives? Please list all.
Water parameters ââ"šÂ¬Ã¢â‚¬Å“ please give actual numbers (pH and your pH range, salinity or sp. gr. & range, temperature range)
Do you see any of the following in your system: hair algae; micro algae; cyanobacteria growths; dinoflagellate (zooxanthellae) growths; brown algae; diatom growth; slimes; colored patches on rock or substrate; etc.?
Maintenance schedule. What have you done lately?
Some help on proper nutrition can be found here:
http://www.reefcentral.com/forums/showthread.php?s=&threadid=785228