On similar surfaces that are immersed, I find that I just have to use a coarse scrub brush on a handle to loosen/remove the algae. Immersion makes the growth soft and easily scrubbed off. But immersion also means the film grows back over the summer, although slowly. When you feel the slipperiness underfoot, you know it's time to scrub again. It takes but a few minutes, so it's something you can do several times over the season without thinking about how long it takes.
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