is very bad about absorbing moisture from under and creating a mold or bloom that eventually surfaces to the top.
needless to say, the tiles you have are not sealed or glazed over?
Do you have a slab or pier and beam foundation?
Is this on one or more tiles? Are the tiles affected all together or contiguous?
Many tiles of the age of yours are not sealed or are very porus.
I would say use some bleach in an isolated area and see what happens. Be careful.
fly your flag
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