My zoas are also disappearing since I got some frags at the swap. I found a nudibranch today on my zoas but couldn't catch it in time.
I'm not sure why nobody has suggested it but the only cure I can find is FLATWORMEXIT by salifert I will be treating my entire system now weekly to kill all adult nudi since it doesn't help w eggs. Anyone have experience w this product? ( i dip coral rx before introducing all frags but I guess I had some eggs somewhere)