Sinularia tree coral just broke apart

[antigonus]

My green Sinularia tree coral had been looking kind of bad were a week or two now, just today they the larger branch literally just broke away, its base seemed to be black and 'rotten'. I left the smaller branch which is still connected to the main base in the tank, as it looks healthier and I hope it can pull through. I did water tests, and even brought water in to my LFS for a second look but they also said everything seemed fine other than the Nitrate being a little higher.  

My parameters today were as follows: Salinity: 1.023, Ph: 8.1, Dkh: 9.5 Ammonia: 0 Nitrite: 0-0.005 Nitrate: 3-5

 

Like I said, I noticed it had been looking rather shrunken for a weeks or two but thought maybe it just my Blenny picking on it, and in a post I made the other day I wrote how my Clove Polyp also seemed to be dying. 

 

Has anyone experienced this before with their tree corals?