Well that's what it is about zoanthids. The long term success is far from garantee...
I had the same thing hapening in my aquarium and many people experience the same thing. I have some paly (pink and gold) that are absolutely undestructible and I can thear them in 2 and both half of the polyp will survive, but the smaller zoanthids are a pain.
I have started to dose vitamine C because I was not ready to lose all my zoanthids and after 2 weeks of dosing, things are getting a lot better! I can see a clear improvement. Some zoanthids that were prone to melting have multipled and the plugs are full of polyps.
If you wish to keep zoanthids for a long time, and you are having problem with melting, consider dosing vitamin C as it will help a lot. First check for parasites, including the large amphipods wich will eat zoanthids flesh, and if no parasites are found, then I would dose VC.
Another thing that I find help a lot to make them grow is feeding Chromaplex. I have tried many type of food but have best succes with this. Although Chromaplex does not prevent zoanthids from melting away.
Also you can try dipping your colonies in 1/3 peroxyde 3% and 2/3 tank water. When ever my zoanthids are invaded by algae, or are closing up, I dip them for a few minutes with that mix. Does wonders
I use Aiptasia X all the time in my colonies because my dear copperband won't touch aiptasias...and it never ever affected negatively any of my frag. Sometime the white paste splash all in between the polyps but they are fine once the stuff is gone. I doubt that is what is killing your zoanthids. Zoanthids are very sensitive to swing in PH, alkalinity, salinity and temperature. Be sure that your parameters are stables.