I'm about to set him "free" into the ocean
That is highly illegal and highly irresponsible to set a non-native species free in the local waters could have ramification on the local ecology that are detrimental. Destroy it before you set it free. The fish has a condition called HLLE
, it is not known exactly what causes it, it seems to be a serious problem for tangs and angels. I have had success reversing the process by improving the fishes diet, and soaking it's food in additives high in [acronym="Highly Unsaturated Fatty Acid"]HUFA
[/acronym]. Improving water conditions will go along way towards stemming the pregression and possibly reversing the process. Some also believe it is caused by stray electrical current in the tank, so if you don't have one, get a grounding probe.