You should have a warranty with the builder. Use it now.

Mainenell my bad husband told me durock used instead not sheetrock bbut can't update post for some reason now.. So is there further waterproofing supposed to happen on the durock? Thanks for your insight. Mauretania yes there's warranty so much work contracted out, this will through company I want it all fixed rightright the first time and not just discolored tiles replaced if there's hidden source mold somewhere want to know what to ask them about, where to look etc I also still have no idea white ceramic tile change color under the finish. Months later

Did they use anything on top of the Durock to waterproof the shower? How is the floor of the shower waterproofed? Did they do a flood test before installing tile?

When your replacement shower is being installed, take pictures of the whole process. Photograph the canisters of whatever they use for waterproofing, grout, caulk, and every other thing. Document it all!!! You should insist on some different sub to do the demo and new shower.

Ask them about waterproofing standards and local building code, and which waterproofing method they will use. Then you can go ahead and view youtube videos of that method of waterproofing installation, so at least you have some idea of how it's supposed to go.

Now, you also need to be concerned about the other showers/tubs in your new build.

There are so many threads like yours in this forum. Look through them, and you'll see. Good luck to you.

Multiple possibilities but without seeing what they did under the tile, we can't say for sure. One guess is the backer board was set into the pan which starts a wicking process up the bottom of the wall. The moisture is what is changing the color of the lower tiles and yes, it stinks (mold).
Make sure you ask them how the entire shower build conforms to industry standards (as stated in this book). If they are unfamiliar with it they are not professionals. Check the qualifications of their tile installers.

Dragonfly Tile and Stone Works, thank you so much for that insight! Will ask them about all that. It is durock behind the wall at least the builder said but am wondering about the floor bc it definitely seems to be most heavy from there. It is a tile and flooring company but they have very poor online reviews and we did not get to choose them as they are already contracted by builder. Thank you so much for that insight and things to check with them on. Have you seen/ heard of ceramic tile changing color like that? It's definitely not on the surface and my husband said it couldn't show through the back bc it's ceramic tile. Do you have insight on how mold can color change glazed ceramic tile like that? Thanks so much for your comment, and everyone else thanks so much for the input as well!

Thank you so much MongoCT that was so helpful!!

Just an info follow up regarding Durock on the walls.

Durock, as you know, is a cement board. Durock itself will not be hurt by water. It can undergo repeated wetting and drying cycles, and it remains what it is, cement board with a rigid structure.

If Durock gets wet, it can hold a certain amount of water. Just like a sponge can hold some water but not drip. When the shower is not in use, the Durock will try to dry out. If it sees excessive water, the water can permeate the thickness of the Durock and moisture can get in to the framing bay behind the Durock. Just like if a sponge gets too wet, water can drip out the bottom of the sponge.

Most installers will put waterproofing on the front of the Durock. It can be a "paint on" membrane like Hydroban liquid or RedGard, or a sheet membrane like Kerdi or Hydroban sheet, etc. Those are the most common ones, though there are others that perform equally well. Then the tile goes right on top of the membrane. Using a "surface applied membrane" like one of the previously mentioned is a great way to control moisture within a shower. Only the tile, grout, and thinset will ever see water. The cement board will stay dry as a bone.

If a surface applied membrane is not used, then an installer will usually staple 6-mil poly plastic (or perhaps tar paper) to the studs, then the Durock goes over that. That method is not as good as a surface applied membrane, and it can have a few shortcomings, but it still works and is better than nothing.

Not all locales are the same. Not all builders build the same. But if an argument is made by your builder that "Durock is waterproof"? Well, it isn't. Just like a sponge, Durock may not be damaged by water. But it can allow moisture to pass through it.

There is NOTHING wrong with Durock. I'm not criticizing it at all. Showers simply need proper detailing.

You said the shower is over the basement. Can you cut a hole in the basement ceiling for inspection? Or in a closet? Or in an adjacent hall?