Preslope not correct maybe, allowing water to pool? Definitely get them back, but sort of waterproofing system was used?

Did they use white thinset?

It looks like your tile is not drying in some areas but I will not assume to know why. The yellowing may be product related not install for my kitchen counters have a few "rust"spots by my sink. They are from a cleaning lady using a not approved product to clean them with.

Provided the shower was properly constructed and waterproofed, this is the natural 'feature' of marble. Don't be surprised when the darkened, wet area continues to grow up the walls, over the curb and outside the shower. We've seen this many times before. See pic below. When a customer fails to heed our advice and insists on a marble floor, we always use a surface applied waterproofing membrane to eliminate the 2 inch thick drypack layer below the shower floor tiles which acts as a huge sponge. Tiles installed over a surface applied membrane dry out faster. Even with a surface membrane, marble will still be marble and suck up water.

Is the darkening in any way bad for the marble? Or will it just repeatedly darken and dry? Moving forward, given that we aren't going to rip out our brand new bathroom, what's the best path?

Looks normal.

Thanks, Creative. And by "normal," you mean you wouldn't be concerned if that was your bathroom? And it shouldn't affect the life of the shower/tile?

It will darken & dry unless it is continuously used before it dries. Then it will grow as C&R stated. Adding more sealer will also slow the drying process. As long as the shower waterproofing was constructed correctly there should not be concerns for damage. If you have in progress photo's of the waterproofing. Now would be the time to post them.

How was it waterproofed? Custom pre slope, not a Kerdi pan slope?

Hello,

Bellow is the link to my recent article that summarizes the research we have done on the subject of marble moisture discoloration on shower floors:

https://www.houzz.com/discussions/5956434/marble-moisture-discoloration-don-t-blame-the-stone

Do you mind sharing what ever happened with your tile issue? We recently built and used honed marble mosaic tile on the shower floor. The tiles all looked uniform color and finish when we moved in. The first week of using the shower a section started to look cloudy. I hadn't even used any cleaners on the tiles yet so I can't blame it on improper cleaning. I have attached photos. I've been told by our builder this is normal, but I don't like the two toned look. On the picture the area of concern is around the drain and up towards the top left corner of the shower. We experimented and let the shower dry out several days and it still had this look.