I have a big toadstool in my tank and when I first got it, it did the same thing.
I asked one of the Marine bioligists at my LFS and he told me that they will close up like that to shed a top layer of skin(Clean themselves).This could take anywhere form a week to three weeks.
Mine stayed closed for about 2 WEEKS then opened bigger and better than when I got it.
It will look like a waxy film peeling off of it, and when that's done it should open in a day or so.
Mine is in the process of doing it again (4 months later) and it's been closed for about a week now.
Nothing to worry about.
PS
Mine is directly under a 250w MH with a MaxiJet 1200 blowing almost directly on it and it LOVES it.