Reefing — help coral won't open

Does your tank talk to you?  Mine does to me all the time; you just need to be able to interpret what it is trying to tell you. Just recently, I noticed a problem: all of my Bubblegum Digitatas were retracted, but everything else looked just fine. When you have 1000+ frags, some will be closed whenever you take a look at your tank, but in this case 30 of one specific type of coral had all of their polyps retracted.  All my other Digis were perfectly happy, as well as everything else – but those 30 frags were telling...