They don't look glazed (although it's hard to tell in the picture).
They look like quarry tiles, which are somewhat porous.
Can you scrape any of the white stuff off with a blade?
I would collect some and see if it dissolves in a drop
of vinegar, which would imply it's salts carried up in
moisture, and crystalised out as the moisture dries.
Another thought would be that the tiles are on something
which is rotting, and it's mold which is coming though,
or mold is growing on something which has been spilt and
soaked in to the tiles.
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