Call the installer back. You should not be addressing this issue. Once you touch it you own it.

This happened to me. I bought a sealer/stainer that seems to have worked.

You have a shower that, simply put, wasn't done properly. Either there is a drainage issue, the weep holes are obstructed, the area around the curb doesn't have a proper slope to drain or they simply put the rubber liner flat on the floor with no preslope. Any and all of this dictates a rebuild of your shower. I see this often, and it reflects badly on those of us that do a proper job.

Now, you are looking at a rip out of, at minimum, the floor...and, if you'r lucky, a row or two of tiles on the walls to tie in the (if any) waterproofing. Folks say, "I got a good deal." Well...you DIDN'T if you have to rip it out and do it properly.

Most likely the pan is leaking/ water proofing issue (as indicated above) may not have lapped membrane system up/over the curb, among other things. If so, must take out pan and install new.

I don't know if this Changes anyone's answer, but the floor discoloring is not traveling. It is contained within the shower. There is a reflection on the glass. The bathroom floor is a larger square format tile.

And it doesn’t seem to lighten or “dry” out...even with hot air blower going for hours. when shower stall floor is wet you can’t tell where it is...then the rest drys and these markings remain.
My contractor is stumped. He thought it might be some chemical reaction to the penny tiles.