Try using a grout or grout haze remover, DuPont makes one.

I would try no chemicals first. Elbow grease.

Ever wonder why contractors are so strong? It's because the job is hard.

introducing a strong chemical cleaner into that glass install is a bad idea. Sorry Carol but I think your off base here.

I've heard mayonnaise or toothpaste also clean up dirty grout.

Mayonnaise? - that is weird.

Toothpaste? - is a mild abrasive cleaner.

If all else fails go to your local art supply store and get a prismacolor permanent marker the exact color of the tile. carefully color the grout spatters. It will soak in an last forever. be careful not to get any between the tiles.

I have the exact same problem in my new kitchen. It's killing me.

Unfortunately, elbow grease seems to be the answer. I've been literally chipping away at the grout with a razor. It does work. But it's going to take forever.

I like the above idea of "coloring in" the grout on the glass tile.

Good luck !!!

Oh. I forgot. When the grout was newly applied, straight white vinegar worked. Stiff brush & where vinegar. Try that.

If it is a cement based grout then a mild acidic solution like white vinegar would help possibly loosen. Cement grouts don't reach their full cure strength for 30 days. So the sooner you get on the grout the easier and likelier it is to clean from the pores of the tile. Glass is impervious so you should be able to clean the residue out if you soak the area and scrub. Unsanded grout doesn't typically like to stick to glass as well as other surfaces. It will take some work but I would use the least invasive method possible.

just make sure you don't use any wire brush or similar that will scratch the glass

Since this dilemma was 8 months ago, wondering how the OP got the grout off. Did you call the installer to come back and remove? Very unprofessional job, if they left it like this, IMHO.