They look like zoa eating nudies to me. You must kill them all if you wish to keep zoanthids. Dip your zoas in RO water (make them close up first) for a minute or so and the nudis will disintegrate but... you will need to inspect each polyp for a spiral shaped string of nudi eggs, pick those off with tweezers. I treat my zoas before placing them in the tank to prevent nudis from getting in my main reef. Good luck and act quick.