superglue is safe to use, it works best if you pull the rock and coral out of the water for a couple of seconds, glue them, wait a couple more seconds, and then put them back, but if you can't remove the rock then pull the coral out, glue the under side, then stick the coral and the glue underwater by the rock and squeeze some glue on the rock, then press the glue on the coral to the glue on the rock and hold for 20 seconds.