Probably ick. Do a google search for ick, and you'll see some pictures of fish with ick, so you'll know what the symptons look like. I hope he/she get's better!
~Taylor~
Probably ick it looks like sand sprinkled on your fish if so there is a medicine called ick away that treats 400 gallons it comes in small bottle and is blue it should help your fish within a couple of days.