Plumbing flux is causing "mold"? That is a first.

During construction a house can get dusty from construction. Add to that the fact that mold spores are pretty much everywhere. You could have had spores and a light coating of construction dust on the surfaces in your bathroom. Add moisture from using the new bathroom, and you can get a bloom. But after cleaning and use, it should all go away.

You wrote you have a "mildew proof tile" so I'm guessing it's a glazed ceramic?

Do you have any photos of the "mold?"

Now lets set the mold aside for a bit and move on to this being more if a water imbalance.

If water is allowed to sit undisturbed in new copper plumbing, sometimes you can get a chemical reaction between the water and the new copper. It doesn't happen all the time, it does depend on the composition of the water. The precipitate that is formed can adhere to the inner walls of the copper tubing.

When the plumbing is then used, the precipitate can slowly flake away as water flows through the new plumbing, it can stain fixtures or tiled/grouted surfaces. That staining usually does decrease and disappear over time as the interior lining of the plumbing gets scoured clean with use.

Mold is one thing. It can form anywhere, even away from areas that get wet from direct water spray, because it's an "environmental thing". Mold spores plus a moist environment can give you mold or mildew, even away from where water hits a surface.

Stains from dissolved or precipitated minerals is another. They usually form where the water directly hits a surface, or where standing water occurs. On a plumbing fixture, or at the base of a wall where the floor slope may not drain well, or around a drain, for example.

So take a good look and see if it's a mold growth or a mineral stain. Mold should come off with a soft scrub cleaning. A mineral stain may not.

You could also have your water tested, especially if you're on your own well. If you have any water treatment in your house, you could have the settings on that checked as well.

Good luck!

mongoct. thank you so much. this is just the information I need. I will post photos when I get home from work. thanks!

Mongoct, a painter, a tile salesman both said it is not mildew. the painter was sent by the contractor and he scrubbed the tile with something and said to allow 5 days to dry. It's only been one night but it looks pretty dry and the dark areas are even darker now. take a look:
you'll see obvious corner side areas but there are a few random very small areas toward the center.

here is another area that shows the wall/floor meeting and significantly more dark area. also note, the grout is below the shower floor level. thank you

It could be that the drain pan is not sloped correctly, and the water is pooling in those two walls instead of going down the drain.

Use a level to determine if the floor goes toward the drain.

Pour a bucket of water in the shower - and see where the water pools.

mongoct - does this look like minerals? I don't think it's mildew - I think it's either minerals or hotmop. coming through. could it even be hotmop?