Okay so at this point in my reefing experience, I am a strong advocate for dipping zoas in 3% hydrogen peroxide for 2min maximum. I have does this to many zoa frags (one colony a couple of times) and it always ends up helping them out. They don’t always look happy right after, and may even form some bubbles under their flesh for a day or two, but just give it time and don’t stress! After maybe a week, your zoas should be looking happier and opening more! The algae on the frag itself should be gone by around day 3-5 depending on how severe it was. Just make sure to rinse the frag and coral is some tank water, after the dip, and before placing it back in your tank!
Most people ask about diluting the peroxide, but if you are using 3%, it’s not needed. Please also know, this is not safe for all corals. Zoas have never had a problem for me, however.