Yeah you can use the hydrogen peroxide, it won't harm the zoas. 1 part tank water to 1 part H2O2 for 3-5 minutes
I'm going to have to side with Dave though. Now that I have an army of snails at my disposal, I would just put it in a small holding container with 10 snails lol, or just put it in the tank. I'm actually kind of worried about them running out of food way more than I am about having algae on some frag plugs.
If its just one plug, it may have not been cured properly