For me it seems to be irritated.
These may be irritants in my opinion:
1-Low Fe/I -other corals bail or not grow at all (but this is definetely not your case)
2-Low PO4
3-Low NO3
4-Low Salinity -> low Mg/Ca
5-Too high light (over 150par)
6-High amounts of pests
7-This is rare aswell but bacterial issues, mostly infection. In this case they melt away, and should be dipped.
Sometimes the skeleton turns red/pink (not cyano! if you can blow it off it is cyano or dino)
Take a look at 2-5, those are the likeliest issues.
This is a healthy head:
-Acan skin is always gray (if you can see it)
-A healthy acan is always puffy
This is dieing:
White skeleton usually means fresh death, since algae did not have the chance to grow there.
But as you can see in the 2nd it's green, which means that the parts died off a week or 2 ago.
For more help I can recommend these videos:
(from FragboxTV)
and this:
(from TidalGardens)