I have no idea why but I have seen this before with euphyllia corals and it was not a issue that led to problems. Wish I knew more and could offer some advice. I know @ZapataInc had a a hammer that was fragged in my tank and when he added it to his, the tentacles did the exact same thing, really blew up.
This is how the hammer looked in my tank. Its weird because once i moved to my new house. They started to look normal. I couldn't figure out what was happening. They didnt seem effected by them "blowing up" and they continued to grow and seemed happy, so i let them be. Also i could get my BTAs to bubble but again after moving all of my BTAs bubble.
I will check to make sure the flow is sufficient. It is sitting next to a torch that has regular tentacles, but you never know. Corals all seem to have their own unique personalities.