A brief read this morning on salicylic acid and alga growth. It can stimulate growth or somewhat inhibit it. It can stress alga so that they make and accumulate commercially useful chemicals. At high doses it can be an inhibitor. I did not look up references on its metabolism by bacteria to see how long it would last in an aquarium. GAC might remove it but possibly too water ionic to be removed by skimming.
Probably another product in a long line of anti-alga products with claims that might be difficult to replicate.