If your zoanthids are in trouble, look closely for a creature that like the one above. Amy discovered this on her sick little zoos, after reading this article by Ron Shimek. Upon further discussion in a thread, she learned the best way to remove these from her tank was to put the zoanthids in quarantine. Once they come out in the open, remove them from the colony, but continue to observe the colony for weeks or months in case babies appear later. Hopefully you will never find a predatory Pycnogonid in your tank!
Above photo by Amy Larsan