I believe this is just residue left over from the application of the grout. You could call your installers but it's likely they just didn't take the time to rinse off the grout better.

First try to wash off the grout film with water and a big sponge. If that doesn't work, which I think it should, then look into buying cleaning products made for marble. Or try just using a straight edge razor, gently. The grout residue should come off.

Let us know how it goes.