y palys do this from time to time. i thought it was a light issue as well. now, though i believe i will likely move them a little higher, i believe they are ridding themselves of sediments \ sand. i say this because the umbrella behavior had me concerned and no growth for a little while had me doubly concerned. but now i can count 6 new polyps in the last couple weeeks while still displaying this behavior.
my 2 cents\experience
I target fed them mysis shrimp. Maybe thats the reason why. Also I target fed everything in my tank mysis shrimp 2 days ago and a couple polyps on some of my frags still have not opened yet. Its so frustrating sometimes when they don't open up but patience is key in this hobby I guess.