It should not be showing ammonia. Has it ever tested 0? Did you do a water change recently? Add a bunch of life or feed a lot? Your nitrites are zero, so either you have a new spike or your test kits off.
If you have an ammonia spike you have a bigger problem. Need to sort that. If you don’t, you might hack that dead head off to stop it from spreading. Take it out, use a chisel and, minding the other heads and flesh, break that piece off and discard it tonight. Maybe suck the dead off it before you try to lift it out. Want to limit the spread of any infection.
As far as the goby goes, he almost certainly had nothing to do with it and is messing with the decaying coral. I mean, it’a be a first in my 20 years if it did.....