six lines and or yellow coris are excellent on red flatworms, but...thats not what the issue is.
I do not see nudibranchs in the pictures, i do see folded edges of the polyps, but not a nudibranch.
When my zoas disappeared it was due to a crab coming out at night and eating them. Have you tried to look at night and see what is scurrying about?
I also have had fish eat zoas as well, but i doubt that this is your problem.
P.
O yeah and for anyone else with this problem, or any coral problem for that matter. I found this very helpful http://www.coralpedia.com/gallery2/main.php?g2_itemId=384