How is the shower pan constructed? Are these "stains" on the entire shower floor? How does the shower drain? What type o grout was used?

Bleach will damage the marble.

I don't know what you mean by "tiny black spots"

It appears you grouted the entire thing in black. Stand back and take a picture. Or several. Are you certain that isn't mold?

If you look closely at the first photo, you can see black spots in the gray grout. I would guess mold.

What color grout did you use? Did it used to look black?

Thank you all! Yes, it definitely looks like mold —the little black dots when you zoom into the picture, but they are not coming out. I know you are not supposed to use bleach but I was desperate because it wouldn’t come out and it didn’t with the Clorox anyways. I hadn’t thought of the pan construction and water getting trapped underneath but that is an interesting idea that I could ask our contractor about. I am not sure how it was constructed or the drainage. Thank you!!

"it didn’t with the Clorox anyways."

No but bleach is not good for Marble or the grout as it will break them both down.

I smell the RIP..........WAH.

I think definitely mold. It's going to get more than "interesting".............