I know you don't want to move all the rocks, but if it were me, that's what I'd be doing.
We're not talking about a small fish like a clownfish or anthias dying, this is a Hippo Tang and a larger one at that. I don't think that small brittle stars and a couple hermits is going to be able to keep up with the rate of degeneration. Also, the fact that this is in such a closed in area is going to work against you, it probaly doesn't have such great flow which will just make the rotting process worse.
I would do some hefty water changes, keep that RO unit running for the next few days, you're going to need it. But honestly, just move the rocks and get that fish out. This is a new tank, right? Why take the chance of fouling it up right away.