I think you can rule out HLLE. If this looks like a separate organism and not part of the fish; I'd be tempted to try removing it with a tweezers. Hold the fish in a wet towel, it may come right off. If done quickly, this shouldn't bother the fish at all. If it does come off; swab the area with a little mercurochrome or something similar. There are many parasites in the ocean that we don't see often, this may be one of them.