Many corals (including zoos) close up at "night" when the lights are off. Conversely some open up or extend feeding tentacles at night. So it's probably OK. However, 5-7 hours is a pretty short photoperiod, particularly if you don't have any other lighting (i.e. additional PC
or T5 bulbs). I'd suggest a timer for the light so that the photoperiod is consistent, and also a slightly longer one, although I don't have experience with MHs, so I'll let someone else who does chime in as to how long the light should be on.