I have used H202 many times. About 2 mil per gallon. I have lost leaves but have not ever lost a plant completely. Now I have no idea if it would have an effect on snails or other critters from the wild.
I pulled out my drift wood which had brush algae all over it and painted it with straight H202 and let it set for a while before adding it back to the tank. The algae got sick but did not die. It didn't hurt my Java moss at all either. I even soaked it in a bucket three times with H202 (3 mil per gallon) for 45 minutes a week apart trying to kill off the brush algae. It only got sick and came back. And still my Jova moss did not get harmed at all.