I have a customer that has a light colored marble floor that she said will dull on its own. (see attached photo)
Is there any chance that the sub-floor was done wrong and is leaching something up through the tiles? 
If yes is there anything we can do to test it or to slow it down?

They have been moping with a neutral cleaner so it should not be the soap, but...

Any ideas would be appreciated. 

Sincerely,
Dustin Fox

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Vallarta Stone Care
Puerto Vallarta, Mexico
619-819-6762 (US)
322-297-2670 (Mexico)