Sigh this is why it is better to let sleeping dogs lie. It looks to be grout staining. Was the new tile sealed before install? Before grouting? Marble loves to stain. This could be marble doing what it loves to do = stain.

Did you tell the new guy to seal it? Not every guy knows this.

Yikes, he should have never tried to polish a piece/section of marble mosaic.

Been there done that, cost me a bunch of expensive tile and my contractor a re-do of the work.

I'm not sure what he did. It doesn't look polished. I would use diamond honing to try to remove the surface but you can try sandpaper to gently sand those tiles. It will be tricky though. Try, starting at 120 grit then 220, 400 to see if it blends.

OR as SJ McCarthy indicated, sometimes it's best to let sleeping dogs lie.

Thank you for these thoughts - replacing that one bum piece was definitely the wrong call... Now really kicking myself for letting him “try to” fix it - not only does my floor look worse aesthetically, but my radiant floors underneath are ruined... This “let sleeping dogs lie” lesson is going to run me into the thousands.