Torch Coral Disease?

[RDNanoGuy]

I would like to hear anyone's opinion on this. I have had this coral in my tank for over 6 months and it has always been healthy until two days ago. When examining the tank at lights on I noticed this one head had its tentacles retracted more than the rest of the coral. Over the next 24 hours the tentacles completely retracted and a white mucous membrane started to form. Today the mucous has covered the whole head. Is this a disease or rot or is this part of the corals life cycle? All the other heads seem fine. What should I do about this?

Any advice would be appreciated.

Thanks