I don't think those polyps were the cause, yes they're toxic but usually not engaging in chemical warfare, I'd say that would be the "leather" coral you have in there. Are you running carbon? I'd give it a try see if it helps.
If it's been a while, more than a few weeks, I'd say change it out. Have you changed anything about your system or husbandry practices shortly before it did this?
Sometimes corals look like death when they split, so that was likely the cause for you! Keep an eye and keep us posted if it changes again as that indicates something else is potentially happening.