It can be so many things, not necesary zoa eating nudibranch. But they might still come back. I had some that were closed for a month and they are now beautiful and multiplying.
In my aquarium, dosing vitamin C is starting to make a difference, but I still have to experiment more with it as I only been dosing for a week and half.
I had some problem zoanthids that recovered when putting them in more flow and more light. If the polyps are very tight together and with very short body, then they probably were in lots of light. If they are further apart and have longer body, then they were probably in less strong light. Try to give them the same light they are used to.
In nature, some zoanthids are exposed to extreme conditions, they get exposed to air while in low tide, extreme heat due to the sun, extreme fluctuation with PH due to freshwater in the rain falls etc...and they can recover even from large bleaching event. Yet in our aquarium they can melt for no apparent reason, with no parasite present.
I have noticed that in my frags, there is always tons of little bristle worm and when I dip them in peroxyde there is usually something like 12 to 15 worms in just one frags. Problem is that those small worms dig their way underneat the mat of the zoanthids and I think they are causing a stress or maybe worse, eating that away.? Now when a frags is having problem, the first thing I do is to dip in 1/4 peroxyde and 3/4 tank water, and remove all these pods and bristles that are hiding there, then I put that frag on my rack and in good flow. It usually come back.
Some are going through cycles of shrinking and being big and beautiful, not sure what's going on there. They do love stability in parameter and any swing in PH, KH and temperature can trigger a melting event. Try to keep your parameters as stable as possible. Also a good food to make them multiply (verified twice in my tank) is Kent Chromaplex. When I feed this in my tank my zoanthids multiply much faster. I tried this in my nano and in my main tank and both had the same results. I was going to also try zoa power if I can find that.
This is not related to your problem but if you like zoanthids it is a very interesting article to read:
http://www.garf.org/FRUGALREEFER/fruzoa/zoaring.html