mystery sac

I have a polyp which fell off the rock it was attached to. Since I was out of glue, it's been sitting on the bottom for several days. Today I picked it up to glue it back to the rock and found this strange sac. It looks like little coral-like flowers inside. I don't have anything in my tank that looks like it.

Does anyone know what's going on?

When this thing changes, I'll post what it is.

I wonder if it's just polyps that something decided to cover with a membrane or if the polyps formed a membrane to protect itself from the sandy bottom where they were resting. The sac was the portion that was in the sand.