WTF is that!?!

[Reef_Geek]

yeah, WTF is that?

if it were me, 2 choices

1) keep my fingers crossed and do nothing. Worst case scenario, it's a sunk cost of $50. Doesn't look like anything too contagious. Doesn't look like lymphocystis which is typically at fin edges and mouths. Might have scraped himself on something and got infected. Likely bacterial/fungal and you don't want to use medications for either in your main system

2) catch it and keep in an isolator (like a net pen, a floating tupperware box with lots and LOTS of holes), and daily, grab him in a net, and use a cue tip to smear the spot topically with a fungicide like mercurochrome (get at your pharmacy, use at your own risk, some species less tolerant than others). Rinse the fish in a bucket before putting back into tank, don't want medicine in main tank. Good luck with catching it. It took me to 3 hours to catch mine.

OR (I don't do this, but some people do)

3) hospital tank with full out in-water medication