Any suggestions for next time?
Euphyllia glabrascens is the least hardy of all the Euphyllias. I have to say this is a pattern I have seen many times with many of the Euphyllia sp corals. Just as a WAG, I would surmise some type of parasite, like a gall crab (one of the theories as to the decline and death of so many Elegance corals) or a nudibranch, or ...
From your description of it's decline, it was obviously not water quality issues, that would have effected the entire coral at the same time. Another possibility is aggressin from neighboring corals, however, the same one head at a time decline, doesn't totally support that option either.