Squirt it right on top of the algae, not in the area. Peroxide’s not a dangerous toxin that lingers, but it’s reactive oxygen that disrupts bonds. (That’s its sanitizing super power.) It can chemically burn if left in contact with skin for too long, so I wouldn’t risk using it on hardscape inverts are sitting on or too near your fish. (The burns aren’t serious for humans but I don’t know how a fish or snail would do before the peroxide dissociated.)

If you need to, put some tank water into another container and treat your hardscape in that so you don’t risk anyone’s safety.