Yeah, it's a zoa eating nudi. Not good. You can get a wrasse, some will eat the nudi's, some won't. It's kind of hit or miss. The only "for sure" way to be rid of them is to put all of your zoas into qt, then dip the zoas every few days to kill the nudis. Since they lay eggs, you will have to qt and dip for quite a while to be sure they are all gone. In my experience nothing will kill the eggs, you just have to wait for them to hatch........then the dip will kill the actual nudis. Good luck.