Bad, who knows what else has been added to it? What other things that a reptile needs, but corals don't.
Bad, could really screw with pH, alk, and cal. Since you don't know what else is in it.
Good, can't think of anything that would be any better than a calcium supplement designed for reef tanks
IMO, I wouldn't risk it. You have spent good money on quality corals, why chance killing them to save a few bucks?