I think my best final answer is this, something from the FS washed out over your tile and grout. Natural stone is always going to have a lot of variation and minerals in it and stone, regardless of what we might think about it, can and will decompose over time. FS is simply a layered concretion of silicates (aluminum, iron and other mineral silicates) held together by layers of cement composed mainly of mineral carbonates (calcium carbonate, etc). That's the creamy white substance you see when the FS peels up along it's bedding planes.
Your tile has a very hard, fired glazed surface that makes it easy to clean debris and deposits from. The grout connecting the tiles is essentially a very porous cement. Therefore, whatever washed off the surface of the FS, stuck to the surface of the tile but penetrated the grout and discolored it.
So the only way to "fix" it, is basically to use a grout saw to rip out all the grout and redo it OR to just accept the discoloration as a "natural" part of your pool.
Perhaps the discoloration will fade over time naturally from sun and chlorine exposure....
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Your tile has a very hard, fired glazed surface that makes it easy to clean debris and deposits from. The grout connecting the tiles is essentially a very porous cement. Therefore, whatever washed off the surface of the FS, stuck to the surface of the tile but penetrated the grout and discolored it.
So the only way to "fix" it, is basically to use a grout saw to rip out all the grout and redo it OR to just accept the discoloration as a "natural" part of your pool.
Perhaps the discoloration will fade over time naturally from sun and chlorine exposure....
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