If it is ich, there is honestly only two treatments, and both will kill any inverts, so you will need to remove your fish from the display and put them in QT.
1. Hyposalinity treatment
2. Copper treatment
I myself am doing the hyposalinity treatment right now. It sucks, but I don't want any more fish to die. And it's recommended to treat all of your fish, not just the one with spots.
Some good links I found helpful, if you haven't already seen them:
Myths/Facts aboutMarine Ich
Marine Ich - Myths and Facts
Curing Fish of Marine Ich