Pop eye most likely, treat with an anti-bacterial food. Hope for best, sometimes fish doesn't recover, sometimes the fish is left with an eye that deflates or is left with the bubble. Usual other treatments are metradonizole and or kanamycin.
It's hard to see what it exactly is. I would try to get it out of the aquarium and to put it in a QT. Lower the oxygen level in the QT and copper dose the QT. Copper will kill most parasites or bacteria and if it is pop eye then it could be because of the pressure within the water. To much oxygen.
Is the fish still eating. IF so QT and feed antibacterial food and treat water.
Buy a 10 gallon tank or five gallon with a heater and some air bubbles. Use that as Qt tank. Treat with copper or melafix marine. Follow the directions on bottle. Get your water exactly like the water in your tank. temp ph sg etc. you could even do a water change and put that water in the qt tank and then keep it at the same temp as main tank. You can get it setup for $30 or less tank heater air stone. small air stone. You can get it at walmart if you have to. Just get the treatment at a LFS
I wanted to update everyone - it appears that the guy that scooped him at the lfs must have wacked him or something in the eye as it looks like his eye is no longer there, as if it fell out and is now just a black spot. However, he looks super healthy besides that, swimming around the whole tank, eating, etc. I now have a one-eyed wrasse