Could be Hexamita, an internal parasite. Symptoms include "Hole in the Head" and white stringy poop. More info:
http://www.fishchannel.com/fish-health/freshwater-conditions/hexamita.aspx
Treatment is metronidazole (more info below). You can soak Metroplex or Metro-MS in the fish's food if you also use a binder (ex. Seachem Focus) to prevent the medication from leaching out into the water column.
Metronidazole: Treats internal parasites (worms), Brooklynella; possibly Uronema marinum.
How To Treat - Metro can be found as a stand-alone drug (ex.
Metro+,
Seachem MetroPlex) or incorporated into a multi-purpose medication (ex.
API General Cure). It is best to soak it in the fish’s food; however it can also be dosed directly into a quarantine tank. Use until symptoms (white stringy poop) are gone.
Whether or not metro is “reef safe” is a topic for debate. The general consensus is that while soaking it in fish food
IS “reef safe,” dosing it directly into the water column
IS NOT and should only be done in quarantine.
Pros - Can be soaked in fish food, making it (somewhat) reef safe.
Cons/Side Effects - In my experience, certain fish seem to have an adverse reaction to it. But these are in the minority.