I don't know what this is, but it formed on an E. specimen in one of my own aquariums many years ago. It did get bigger or smaller at times, but persisted for over a year before disappearing if memory serves. I gave the coral to a friend about three years ago, but still see it in his aquarium regularly and the bubble has never come back. If anyone has any ideas, please let me know. No one that I've showed this to has had an answer.