Of course rinse the spot with clean water right away and wipe dry.
Remove and replace grout with white marble tile.
Remove all the grout between the tiles.
As you work be very careful to avoid the edges of the tiles themselves as you do not want to chip or scratch them.
Scrub it lightly with the sponge to remove the haze.
Let it soak for several minutes as directed by the manufacturer instructions.
He told me to wash the walls down with 1 part white vinegar and 5 parts warm water before he comes by to remove all the residue of the grout.
If it is actually a cement haze then if you take some diluted vinegar in a out of the way spot and apply it to the haze it should remove it.
Because it is a polished marble made of calcium carbonate it makes it acid sensitive.
If you attempt to remove a tile while the grout is in place the grout joints will put pressure on the surrounding tiles which may weaken or crack them.
It is not that hard to repair grout once you un.
Using a grout saw or an electric tool such as a dremel with an appropriate attachment begin to remove the grout from between the marble tiles.
This can kick up a lot of grout dust so make sure the area is well ventilated.
Rinse the sponge often and change the rinse water as it becomes cloudy finally allow.
In concert with the grout float a large damp sponge can be handy for wiping off any lingering grout haze.