Once cured it's harmless. A lot of people use it to secure plants to wood and rocks. Make sure it's cured before adding
I usually see it attached outside the water then put in water. It has a cure time of 30 secondsAs previously mentioned people glue coral frags all the time by putting glue on them and then attaching it underwater! Thats the opposite of cured. It surprised me when I first witnessed and did this as well. Super glue actually cures from water vapor in the air. Naturally then it cures underwater quite well too! If you get super glue on your fingers try to wipe it off...not wash it off.