Try a peroxyde dip and try adding a bit of vitamine C buffered to your tank (sodium ascorbate).

I had a few do this and after a week or two, sometimes even a month of looking like pine needles they started to unshrink and get to their full size again. No idea what is causing it, as they can look healthy and have huge beautiful polyps one day and look like that the next day. One thing I have noticed is that they always start this cycle when the metal halide (main light) goes out and the actinic led is coming on, somewhere not long after the actinic come on.

A theory I read is that sometime the zooxanthellea inside the coral produce oxygen and that oxydize inside the polyp making this toxic for the coral. Maybe that is true since vitamin C seem to help and it is an anti-oxydant.

Did you change the light recently?

Another question...did you introduce a clam or do you feed clam, oyster or mussle to your tank recently?

Was there any shift in alkalinity?

What is even more puzzling in my case is that sometime the same coral on the same frag plug, but unconnected with the other, start to shrivel like that and the other polyps that are attached to a different matte are not affected. This is really wierd since it is the same specie of zoanthids, same variety, same condition etc...yet only part of the frag is affected. That has to be something attacking them or a process that is internal and local, either a bacteria, virus or this oxydation theory. It does not seem to be contagious obviously. I now have a frag of magicians doing this..3 polyps that were separated from the main colony are just fine, while the 20 or so other are all shriveled like yours.

I separated the colony in 4 and have put them all in different lighting and flow, to see wich condition help them regain their health better. I am dosing with vitamin C and did 2 dip in revive (to no avail).

If you have zoanthids or paly that you really like, then fragging them and splitting colonies is the best bet to prevent losing them all.

My armor of god pally's did this in my reef tank I moved them over to my fowlr tank and they recovered and are doing well now. The odd thing is both tanks are plumbed together so the water quality is the same, however I have MH over the reef and T5 over the fowlr tank. Maybe move them to a darker spot in your tank and see if it helps.

I would say it is slime algae or something similar like a fungus

do a large water change and brush them with a baby toothbrush imo

Has happen to me alot, it could be a # of thing's, zoapox etc which is from, sudden changes,climate change,water clarity/quality change etc

Not algae. Tank is stable.

I had some of my green ones do this a while back. I siphoned them off, but some of their base was left. After a few weeks, they started growing back again. I have no idea why they did it as I had no tank changes, but there are a lot more of them after they came back.

Pat, my colony of those did the same thing to me after about a year. I had a nice big vibrant colony and they melted away like yours are doing now.

pat try fragging them since they are a close to a goner anyways , splt the colony up and give out a small frag to someone or put it in another tank yourself maybe the mandarin tank if anything you may be able to get a frag back .just a thought;)

Really lost at what you are saying Patrick.
What is the relationship between stability and algae?