We have seen a similar issue in the past. The problem occurred when the porcelain tile on the floor did not have 100% contact with the tile adhesive, this caused water to seep down though the grout join and hold in pockets under the tile in the areas where there was no direct contact. The water being trapped in the pockets under the tile does not evaporate and can leave the tiles looking stained. If this is the case with your floor tiles the bottom row of carrara marble tiles are simply acting like a sponge drawing water up. To fix the problem we would suggest that the bottom row of carrara marble and the porcelain tiles on the shower floor need to be removed and relayed ensuring full adhesive coverage. Unfortunately this may compromise your under tile heating and waterproof membrane. Proceed with caution.

We used the same ones. Attached the pricing. Our builder got them for slightly cheaper than the sale price.

I would go to a tile place Im sure they would have seen them before somewhere -Goodluck

With most old houses walls and floors are not straight and perfect, however wall tiles can go some way concealing this. I don't know if a reputable tradesperson will fix glass panels to old tiles and be happy to guarantee the job. Seems like an ideal opportunity for mould growth between the old tiles and the new glass if any shower water gets an opportunity to seep around the edges or joints of the glass. I think you may be surprised just how much this type of glass costs to install, and you would probably be better off doing the job properly with new tiles and a proper waterproof membrane as required by the building regulations (which your old shower recess doesn't have). In the future your potential new buyers will probably appreciate a freshly renovated bathroom that conforms with the current building regulations (with standard tiling) rather than a glass panel over mould in a bathromm that doesn't comply with the regulations. Best of luck, Dr Retro of Dr Retro House Calls