You know what??? That is soooo weird because my palys that were exactly like that started diminishing, and I had no clue why because they were doing so well! One morning, my larger colony fell off of the rocks and I picked it up and when I did, I noticed a critter in the center of them that looked exactly like a black tick. No kidding. It looked like a tick. I immediately threw the whole thing in the trash. I'm crazy like that... I saw red bubble algae growing on a a few frag plugs of some zoas that I bought from Alti and I tossed the whole frags because I didn't want that crazy stuff to spread in my tank. I know that I could have removed the zoas and glued them somewhere, but I don't have stuff on hand to do that kind of stuff. When I saw that black tick looking thing, I didn't want to pick it off, so I just tossed the whole thing.
I wonder if you might have something that is hiding somewhere within them eating or bothering yours like I did with mine? Because it's just too weird for them to be doing well and everything else is doing well except for those palys? Check them out closely and see if you can see anything "odd" going on....
Sorry to hear about them though!