I've had the same thing with two of my emperor's and I haven't been able to diagnose it. So far, I've had one of them manage to stay alive for a couple weeks somehow. The other one didn't make it.
From what I've been told, a salt bath is never a bad start to try to help the fish. I've never had much luck with adding medications to the water or putting my fish in a hospital tank.
Now that I think about it, I've seen something similar when I had rainbowfish.