I understand you don't want to give up. Sometimes you need to. Zoanthids and palythoa corals have a toxin that CAN harm you. Please take the warning from myself and @A. grandis And Get Rid Of It. Do not put It back in your tank.
I had that same thing going on. It spread from one rock to another over time. I never figured out what it was but adding vitamin c stopped the issue. Although I did add wrasses and other fish to help with pest and decay. Did you save any zoas?