I'd get him back in there to fix it. Hopefully, you haven't paid in full yet.

I'd be leery of chemicals, but this youtube video has a labor intensive but non chemical method:

https://www.youtube.com/watch?v=fhkoAXQiGW0

I sadly did this is the “improved” tile it was a terrible experience and bad contractor /:

Thank you for linking!

Mr. Clean magic erasers and elbow grease. I had a similar problem. It sounds unlikely, but they work. No chemicals, just mild abrasion, and no damage.

Miracle Sealants Acidic Cleaner - it is under $20, a diluted mix of what you need! Hope this helps for an at-home fix :)

Cleanworks: Acidic cleaner on marble? Isn't that a no-no?

eta: their website says ok to use on non-polished marble? Would these tiles be polished or non-polished?

Unpolished marble

Do you have any extra tiles to try the cleaner on to make sure it causes no harm? Why is the contractor not fixing this for you?

Good idea. This IS them coming back and fixing the grout. ☹️🙃

Oh, no!

Have you tried the sponges? I thought my neighbor was nuts for suggesting the idea, but it worked! Nothing to lose at this point.

Hi everyone- tried sponges and hot water it’s not coming off. I’d like to hire a professional for help. Is a tile installation or tile cleaning company best?

I’d like to have them clean and reseal. Thanks!

Magic eraser sponges? I’m no expert, but I think marble restoration is your next option, unfortunately.

Yes tried that too. :( unfortunately yes

Oh, so sorry! It was such a great solution for my problem I was really hoping it would work for you too.