I had the exact same thing happen on a large GSP a few months back. It stayed completely closed up for about a week and then went through a big growth spurt and doubled in size in about 6 weeks. Someone else told me they experienced the same thing (they may have posted on this thread also). I would just leave it alone and give it a week or two.
My tanks: 120 Reef w/L2 Algae Scrubber, 60 Reef Pond w/floating Algae Scrubber, 40 Breeder Reef w/L2 UAS Tester Algae Scrubber
I maintain: 144 SW Reef w/L2 Algae Scrubber | 200 SW Reef w/L4 Algae Scrubber
Special knowledge: Algae Scrubbers, Acrylic Fabrication