How old/big is the FM, what are his tankmates? How long has he been in the tank? What size is the tank and what are the water parameters? Is there signs of parasites, I've noticed a lot of the FM's from my LFS have intestinal parasites, threads hanging down when he poops. If all that is fine he may be stressed, consider adding hiding spots, perhaps there is low oxygen levels in the tank, consider adding an additional airstone or Powerhead.