It's popeye. It can be due to decompression if the fish is located at deep depths and not brought up to the surface correctly, or a bacterial infection. I believe the most common way of treating it is to put it in a tank filled with antibiotics (the drug kanamycin comes to mind, but that might not be the most correct drug to use). If treating within the reef, you probably need to get your water quality back up by doing more water changes.
A more invasive approach would be to sedate the fish in something like MS2-22 or clove oil, then use a very small needle to alleviate the pressure by inserting the needle through the eye socket (not the eye itself!!!), and drawing out the bubble/water within the socket. If you wish, you can then take some diluted medicine and flush it into the eye socket, being careful to not pop the eye out of the socket, and then remove the medicine until the fish' eye is back in position.
