Is this a coral disease?
 

A few days ago and still to this day, I noticed a bubble growing on top of one of my frogspawn head. There are little tentacles growing around it - but is this a disease? Please help. Thanks!

all my euphyllia does that all the time. sometimes the bubble burst and shows the skeleton inside, but usually close up and heal up in few days.

maybe it does that when it's forming a new head?

werid you mention that, my acro plana, just had this happen over night, 2 of these bubbles, the polyps are still attached just streched away from the skeleton.

It's forming a new head