The stronger the leaf the stronger H2O2 is OK, or the longer you can leave it in the dip.
Finer or softer leaves will not tolerate it for so long, or at so strong a dilution.
I would start with the most leathery leafed plant (for example, Anubias) and perhaps just pull off one leaf, one that has the most algae. Similarly, pull off a leaf from one of the most delicate plants, and a sprig of moss. Maybe put them in a mesh bag to keep them together. Dip these test parts in pure (3%) H2O2 for 5 minutes and then toss 'em back into the tank. See what happens over the next 24-48 hours. Even though the leaf is detached from the plant it will not deteriorate that fast. Any damage you see will be from the H2O2. If the algae is dead and the leaf is OK, then that is one end of the limit.
If some of the sample parts did not stand up to that test, then retest, using 50% of the H2O2 + 50% water. Same 5 minutes.
If that keeps the leaf alive, but maybe the algae did not die, maybe you need to use 50/50 for 10 minutes.
Keep on juggling dilutions and durations until the leaves are OK, and the algae dies.
Here is another guide, then my comments added below:
The fish in an aquarium are generally unharmed when the H2O2 (3%) is added at the rate of 1 ml per gallon, and some people have found that even 1.5 ml per gallon is OK.
By the time the dose reaches 2 ml per gallon the fish are not happy.
So... if you want to use a syringe (no needle) and squirt little bits of H2O2 into the full tank, aimed right at the algae, use no more than 1.5 ml per gallon.
But if you want to drain the tank until the algae is exposed you can squirt pure (3%) H2O2 on the algae infested plants while the plants are in the air. Wait for the bubbling to cease then refill the tank. The bubbling allows the oxygen to escape to the air so when you refill most of the H2O2 is deactivated.
My comment: The species of plants were not specified, but it sounds like pure H2O2 for 5 minutes is a good place to start. I am almost sure this was a treatment for BBA. I do not think the species of algae is important. If the H2O2 will kill it, that is what you will find out with a test leaf.