My green one did the same thing, it was near my red and I'm going with chemical war fair.
So far it has been like that for about a month and the tentacles are slowly starting to come back out. FYI, I've not seeing any of the flesh recede.
I got mine from aquatic art and they have been growing that particular one for a very long time. A while back they had an issue with the mother colony and some chemical war fair. Thiers took a few months, but it came back.