GSP Question

[Arcticfirefighter]

My GSP have me scratching my head lately... I have had them in the tank for about 4 months on the same rock, they are isolated for obvious reasons. Three days ago the one on the left retracted and has not opened since, the one on the right fully extends as usual. Water parameters are good with the exception of Nitrates which are at 20, normal for my tank ( yes I am trying to fix that [emoji16])
I guess my question is, with them being basically nuclear bomb proof, is it normal for one to just retract for days on end? All other coral (Softies and LPS) are good to go. Thanks in advance!

 

[DSC reef]

I honestly wouldn't worry to much. When our tank had a large colony on the overflow and rocks at random half of them would close then open days later. I stopped trying to figure it out, lol. It kept growing nonetheless.