You can get a grout haze removing product from most DIY stores to try and get rid of this. We've just grouted our backsplash with a mixture of marble and glass mosaics. The glass has a rippled effect and of course the grout got into them. I found that some hard scrubbing with a sponge helped get it out, but the grout is the same colour as the tiles so isn't that noticeable in this case. I have heard vinegar is good for this, but it might not be great for marble tiles.