The algal growth has gone into the stone and is impossible to remove with cleansers or bleach. Looking it up on the web, it seems as though the best way to deal with this is to sandblast it clean and then give it some kind of epoxy coating like you'd use on a garage floor.
Updates as events warrant.
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One other possibility: At least some of that looks like iron and/or manganese staining. Either deposited by precipitation from evaporating water over the years, or from oxidation of less-oxidized (and less-colored) forms of these metals that had been originally present in the stone. Bleach isn't going to touch them, because bleach works by oxidizing colored organics to non-colored organics, or oxidizing insoluble inorganics to soluble inorganics. Since the iron and manganese are already oxidized, and their oxidized forms are the insoluble ones, oxidizing them further isn't going to accomplish much.
It might be worth a shot to hit it with some hydrochloric acid (or muriatic acid, as the hardware stores call it). If you've got iron or manganese stains, that should strip them right off. If you can't find the straight stuff, there is something called "CLR Multi-Surface Cleaner" that should do the trick.
Of course, if the stone is marble or some sort of cement-based casting, hydrochloric acid will strip the top layer of the stone right off, too. Which is all that the sandblasting would do anyway, so hey, it's a win-win! And it should make a nice, clean surface that the epoxy will bond tightly with!
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