Ouch, That is about the worst case of exopthalmus I've ever seen. I'm afraid that at the very least, that fish will lose its eye. The first thing to do is to try and determine the cause. There are two basic causes of "free bubble" exopthalmia: supersaturation (usually affects both eyes, and usually is seen in more than one fish). Or it could be from mechanical damage. Was this fish captured in a net recently, jumped out of the tank, or otherwise get some trauma to the eye?
People often cite bacterial infections as a source of exopthalimia - but that is actually fairly rare. In those cases, both eyes are usually infected, there is often signs of inflammation, and the eyes protrude as if something is pushing from behind, not showing bubbles right under the eye tissue like in your moorish idol.
I've never been successful treating a fish with an eye that bad, and there really is not any practical treatment that a home aquarist can undertake. The two choices are to use MS-222 to knock the fish out and then draw the gas off with a syringe (but the bubbles will almost always come back) or to use a pressure chamber to reduce the bubble size and allow the bubbles to go back into solution in the bloodstream (but that won't work on bubbles this large).
Sorry.....
Jay
