My Duncan is the most resilient coral in my tank, doing fantastic in a variety of differing flow and light intensity circumstances. In your case it could simply be that things changed. If it stays closed up for more than two days without ever opening up even briefly, THEN maybe I'd start to worry but not before. This just happened with my anemone: I added a new piece of cured live rock and my nem closed up for two days. Now he's back, like nothing every happened.