A lot of corals open up their polyps at night. A big reason is because there is a lot more food out at night. You'll notice that there are a lot more pods out at night. Another reason may be that the tank you bought the corals from had something that bit/ate the polyps, so the coral has learned to open up at night when that fish or whatever is asleep, so it hasnt figured out that that fish isnt there anymore. (i hope that made sense)