Make sure you are feeding them. All protopaly must be fed from time to time. They can survive for a long time without feedings, but my guess is that it is catching up to them. They must be spot fed meaty foods.
I have some large protopalys that will actually catch and eat ghost shrimp meant for anglers that stray too close.
Your water parameters may be too 'clean' for them.
Hello Kigs, sorry to hear about this. I highly agree with flricordia's advice.
If they are all indeed protopalys, and those are the only 3 which are showing signs of degradation, it is almost always a deficiency in your system. You have 3 frags/colonies, in the same system, placed throughout the system, have been doing great for a long time, then all of a sudden the begin to decline with no other manifestation in any other polyps in the tank, no doubt there's a deficiency. This is why I'm a firm believer in testing everything I can every Saturday morning and some call me a nut for doing it as they think it's over killed. I seldom if ever loose polyps, KNOCK ON WOOD, maybe it's luck, maybe it's being proactive, I don't know, I'd like to think the latter .I can't see how the Furan, which works well with gram positive and gram negative bacterial diseases, would impact this condition you are having. A fresh water dip wouldn't work either in this case. Slow, gradual, moderate changes are what I'd advise until you restore them. Drastic corrections can cause already sick/declining polyps to easily go over the deep end when they are retracted, collapsed or on the decline.
The tank stress could very well be an issue or contributing factor for their decline. Slowly reducing that stress could be the trick to bringing them back.
I agree with flricordia, protos are robust eaters. They can survive without food, but almost always look more fully bodied and reproduce faster with supplemental/secondary food sources.
Do you recall anything happening or changing in your system just prior to the decline?
Have you tested your water parameters across the board? This is where I usually start with any unexplained polyp regression.
A quick 20 % water change always seems to help. It did this the last time I recall having this issue and that alone slowly brought them back.
I also placed them on the far end of a current flow of current. I did this so they are constantly hit with less than the normal high current. Doing so still affords them the opportunity to feed if and when they expand day/night when you can't see them. The subtle moderate current is also less stressful on a sick or declining polyp. A sick polyp almost never recovers when being thrashed buy high powered current. The current will also help removing any stress related slim or flaking off of the polyp as they rebuild/recover.
I'd also run the actinics only for 2 full photoperiods after the water change.
Man I really hope they come around for you, it would be a great Christmas present for sure.
Best of luck, Mucho Reef
PS, sorry to go overboard, good luck