It is not that hard to repair grout once you un.
Remove and replace grout with white marble tile.
Rinse the sponge often and change the rinse water as it becomes cloudy finally allow.
Because it is a polished marble made of calcium carbonate it makes it acid sensitive.
In concert with the grout float a large damp sponge can be handy for wiping off any lingering grout haze.
This can kick up a lot of grout dust so make sure the area is well ventilated.
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.
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.
Apply the grout remover to the grout haze on your marble tile.
Let it soak for several minutes as directed by the manufacturer instructions.
Of course rinse the spot with clean water right away and wipe dry.
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.
Etching is corrosion that eats at the marble destroying the shiny polished layer and exposing the more dull marble underneath so you get dull and light discolored spots.
Remove all the grout between the tiles.