“up to $1,800! - to hone or treat the marble.”

Unfortunately, honing or treating the marble will not help. It may look good for a week, and then you’ll start to see the rust again. So honing or treating the marble would be money out the window. Marble itself does not rust. Something made of metal is in there, which is what is rusting; as @User said, likely screws. Improper installation unfortunately. Though I don’t know that anyone can know over the internet whether the whole thing needs to be rebuilt, or just that marble threshold and glass door need to be replaced. Perhaps once the installation errors are fixed, that marble threshold at that point can be honed and treated to remove the stains, and re-installed.

You need to get a bathroom contractor to look at it IRL, to assess. It will cost you what it will cost. Those are the perils of home ownership. Whoever installed that in the first place hired a crap contractor to do a janky job. Don’t make the same mistake or it’s just good money after bad.

That's fastener bleed and you're in big trouble. It's a do-over. Could be moderate or severe.

@ MaryJane. Please explain this for me.

"Since the non profit Ceramic Tile Foundation’s link to ”A Homeowner's Guide to Hiring a Qualified Tile Installer ” isn’t HZZ approved anymore,"

Seeing I have only posted a couple of times over the past years. I introduced that link here after contacting the CTEF directly for approval as I felt it was a great benefit for the homeowners here on Houzz. To this day I continue to contribute my time and efforts in support of this organization.

The Homeowner's Guide to Hiring a Qualified Tile Installer (ceramictilefoundation.org)

Yes @ MaryJane, as Creative notes, curious about where the "approved" HZZ reference is/or isn't? The NTCA is working on a major consumer education initiative that will include CTEF links (NTCA sponsors CTEF). The Homeowner's Guide link Creative refers to is being used at an increasing rate as consumers realize the need for qualified labor. There will be further education tools and information for consumers on both the NTCA website and CTEF.

J C , with respect, we went off into industry references without commenting on your question. We would agree that at first glance from afar, this appears to be not surface, but penetrated from below and as others mentioned, perhaps an improperly built/tiled curb but could even be a bigger problem. Some of these symptoms require what a physician might need to diagnose....it's what is under that tile and we can't see it. We would remove that curb top to evaluate further. Need to see below it.

@Mary Jane, thanks for explanation. Not sure what that problem is but appreciate the info.

@MaryJane It's not from screws... it's from the natural metal deposits in the marble and something that happens often with Carrera.

Did the seller disclose the leak?

"@MaryJane It's not from screws... it's from the natural metal deposits in the marble and something that happens often with Carrera."

Nuh-uh. It's fastener rust and contamination. When you tear it out, if there aren't fasteners corresponding directly to the rust spots, I'll donate a thousand dollars to your favorite charity in your name.

Fred Houston wrote a column about a cheap builder that wouldn't run the air conditioning after his marble was installed. The marble rusted and the entire floor had to be torn out. It did not rust in bright orange spots.