I don't believe anymore that acans mysteriously die. There is always a cause. You need to spend more time with your reef observing and experimenting. Sometimes, its a worm that only comes out at night. Sometimes, its amphipods at night. Sometimes, its a sponge that smothers acans. Could be an bacterial infection or a protozoan. The best advice I can give you is frag away any unhealthy portion. When you frag it away don't cut along the unhealthy and healthy boundary. Cut the healthy portion a lil in from the recession so you make sure the healthy polyps you seperated are totally healthy. I have asked questions like yours to many higher ups in the business. Word from Australia is frag out the bad polyps. Dipping with Melafix might be a good idea cuz it will expose any pests living in its skeleton, my extra .02.
Hope that helps. I can't stand to know acans are receeding. Lets get em healthy again!
Kris