What is happening to my coral?

[DadE02]

Does anyone know why my coral is growing a sudden bubble.

I noticed a small bubble the other day, but it was hidden inside the full day bloom of the coral. Tonight, while the coral is dormant, I notice the bubble is a lot bigger.

See photo below from tonight

 

[Sniperhank]

This is an oddity that I see once and awhile. I'm not really sure of the cause, but I've never seen any harm come from it. It should return to a normal size.