Have you tried a little kitchen sponge with the scrubby on the one side, and some plain water with a couple drops of dawn dishsoap? Scrub it gently, with a wrung out sponge using the scrubby side.

How did it get there...from what?

We aren't sure how it got there. The homeowner says the contractors used some kind of polish and it obviously didn't agree.

I would use some fine (#0000) steel wool and a bit of mineral spirits to remove the white stuff. Or start with a soft cotton rag & mineral spirits...and proceed to the steel wool if just the rag doesn't do it.

I have a very similar issue. What did you try? Did you resolve the problem? Thanks