Hi, hmmmm-
Definately sounds like an ich infection.
There are many schools of thought on how to deal with this. Anything from freshwater dips, to copper treatment in a hospital tank, to hyposalinity, to nothing at all.
If you try a freshwater dip, make sure to match the temp and ph of the freshwater with your tank water. Dip for about 5 minutes, this will cause the external parasites to fall of, BUT reinfestation may occur, as the cysts of ich will still be present in the display.
Only use copper in a medical tank. Follow the directions on the bottle, and confirm amount with a copper test. Never used this method myself. Same problem applies as above. Ich will still be present in the display unless you remove and treat all the fish, leaving the tank fallow for a few weeks.
If this is a fish only tank (no inverts, including corals) You can drop the specific gravity over the course of a few days to around 1.018. Leave it there a few days beyond the total disappearance of any symptoms, and then slowly raise back up to normal range.
If the fish is still feeding and is not completly infested, I prefer to bolster diet with a liquid vitamin supplements like selcon. You can soak the food in this and then feed it to the trigger. Make sure there is nothing going on with your tank that is causing stress to the fish, this often will precipitate an ich outbreak. Make sure all your parameters are within normal range (PH, ALK, Nitrate. You should have no detectable nitrite or ammonia in an established tank.)
Often times following these last steps and simply waiting it out is a less stressful course of action for the fish. However if this is a fish only tank with NO inverts I would recommend dropping the specifis gravity a bit combined with the enhanced diet. This should kick it in the ....
Good Luck! - Becca