These stains, which I guess are calcium have been on the tiles ever since we inherited The POOL. I tried a blue pumice stone design for cleaning pool tiles off Amazon and it did nothing.
If it’s extensive then you’ll need to hire a professional pool tile cleaner. They use glass bead blasting to physically remove the calcium. Some use MaxxStrip (kierserite) media but that is magnesium sulfate and you really don’t want to add that to your pool water if you can avoid it.
That efflorescence suggests that water is permeating from behind the tile. That’s not a great sign as it means it will keep coming back. Sealing the tile and grout might help a little bit it’s not a bullet proof fix.
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