Had you done Kerdi over deck mud I'd say there could be a low spot or a "bird bath" in the corner of the pan. While it's still possible that there could be a drainage issue with a Kerdi Tray, it's a bit unlikely unless the installer did a nasty faux-pas of sorts. As to a leak though, yup, what the others have written. The leak would have to be "inside" the shower to indicate on the "inside" of the membrane. So it could be a void in the grout that's holding water. It could be voids in the thinset under the stone, thinset ridges not fully compressed, that are holding water. It could be the stone itself...a different lot within an order. Lippage between adjacent stones could cause hamper drainage. It could be uneven sealing. Was the stone sealed? Was it sealed after the floor had seen water? When you shower, does surface water drain well from that corner or does it stagnate/puddle at all? If you put a hair dryer or a fan on the "dark wet" marble, does it dry and lighten, and stay dry? Or dry (and lighten) then darken after the heat is removed? Or does it not appear to dry (lighten) at all?

Thank you all for your input! From a cost & speed stand point, I think a pre-formed shower pan is probably what we would prefer. Our shower is a pretty standard size 36"x36" so I don't anticipate running into a lot of trouble in finding a pan that will work. live_wire_oak thank you for your suggestions on other materials for the shower pan, I will look into them. I agree with you that if we had plenty of time, we could probably figure out how to do the shower pan & tile the walls. Our timing may not have been the best for this project as we have a fairly hard deadline of end of March for getting this done. As a result we're trying to do what we feel confident doing and farm out that which we could probably do, but would be massively slower at. Calling individual trades might be another way to go. Hadn't considered that option. I'm willing to GC it myself if that's what it takes.

@segbrown - It certainly could be done during tile installation. This type of shelf is not a after thought. If the tile is not installed simply reduce the size of the niche.

Since they didn't finish the Schluter installation, its definitely not right and it will not seal. However, based on what I can see in the picture it can probably be fixed by a qualified installer. This appears to be Kerdit membrane, and every seam or penetration must be sealed with Kerdi band and thinset. All joints must have a 2 inch overlap. There are also specific Kerdi band products for inside corners and long flat runs, so you will get a better seal. This is one example of the videos that show the proper installation. https://www.google.com/search?q=how+to+install+kerdi+band&oq=how+to+install+kerdi+band&aqs=chrome..69i57j0l2.5983j0j7&sourceid=chrome&ie=UTF-8#kpvalbx=_p6EZX6etEOWwytMPu9m16Ao60 Bruce