The diluted drug store solutions that people are applying to their tanks are probably not harmful at all to their tanks long term. The algae is being killed because of the relatively high local concentration of peroxide; very likely it's damaging the cell walls of the algae who don't have robust tissues designed to protect them from powerful oxidizers. Make no mistake, it could kill your corals too, but coral death likely requires a higher concentration or longer duration of exposure. It's highly reactive and it decomposes in to oxygen and water, so after a few minutes or the application of lots of flow all you'll be left with is slightly more oxygenated water and whatever stabilizers the manufacturer put in it to prevent it from breaking down on the store's shelf (usually an acid of some sort).
I bet that every coral has a slightly different limit on how much they can be exposed to. Since corals don't come with a 'tolerance to strong oxidizing agents' rating, I probably wouldn't dip a prized Acropora colony in the stuff, but it's been used to control algae and cyano on aquatic plants in the freshwater hobby for years and years, so I don't see the harm in local applications to problem algae spots with the pumps turned off for a few minutes.
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