Fish zoanthid poisoning?

I recently added 2 heniochus to my aquarium and today, one is gone. Just gone. I'm assuming dead behind the rock work. Good thing for nig skimmer and CUC. Yesterday the heni in question seemed very washed out but otherwise fine, it had taken to picking at my coral and he was scheduled for removal. Same day I noticed a colony of zoas were closed up and irritated. This is the second time I've noticed bothered zoas and lost a known coral muncher. This happened to a coral beauty a few months back. Bothered looking zoas, next day dead fish. Have any of you had similar occurrences? The other heni is doing great and seems to like to nip some green button polyps which I'm 100% fine with.

The zoa colony in question is 20 or so heads, all were closed.  75 gallon tank, 50 gallon sump, no AC.  Im not referring to leeching in to the tank, but I think the heni took nips at all the zoas on that colony and actually ingested the toxin.  All other fish are doing just fine.  I see no reason that a strong swimming fish that ate 2 days in a row just up and dies.