I've used it and it will disolve the bryopsis and other "soft" algae. 3% is fine. I'd read up on it first. I would brush the areas with a stiff plastic brush after applying the H2O2. I've never seen it harm LPS or SPS but I'd still take care to keep it off coral polyps & give it a good SW rinse when finished, before putting it back in the tank.
Many people may not understand that bryopsis can regenerate from a left over fragment so full tank dosing may be in order in some cases. Also, if nutrients, especially phosphates, are not driven to near zero and kept there for quite a while, it may very well return. Other methods of nutrient control should also be used; mechanical filtration, water changes, prudent feeding, wet skimming & perhaps GAC. Using H2O2 is a long(ish) term project but easy & inexpensive to do.
But it's so darn prolific that IMO your rocks should be taken out ASAP & treated in a QT tank or buckets. Don't give it an opportunity to spread. And then get on top of your nutrients! Good luck.