So. . .You last tested your water more than a month ago. . before you acquired this fish?
Water quality is very important to fishes (and most marine life). You should try and get into the habit of daily water quality checks until you and the tank are well seasoned.
The fish could be suffering from a water quality issue. (NOTE: Not all fish are equally sensitive to different components in the water, so the fact that other fish/marine specimens are doing okay is not verification that the water is okay).
The fish could be suffering from stress issues.
The fish could have a deficient diet. See this:
http://www.reefcentral.com/forums/showthread.php?s=&threadid=785228
Or, there could be things going on in the tank, associated with MHLLE that explain the condition. See this:
http://reefkeeping.com/issues/2005-06/sp/index.htm
And of course the fav. . .The fish could be suffering from a bacterial (or other microbe) infection.
There are many possibilities that can't be sorted out here without much more information and hopefully a photo.
I would improve its diet and include the supplements recommended in the above reference. I would also look towards water quality issues. Do your tests and then a large water change. Repeat your tests after the water change.
If you can, show us a photo of the fish so we can see what you're looking at. We need a fairly up-close, clear photo.
Good luck! :rollface:
