Since you don’t see the purple stain in the pink tile, but you see it in the white tile, and you can’t clean it, it sounds like you have a microbial staining or mineral staining showing through the white tile.

The reason you will get mildew on the tile in a shower is because the water is not draining or drying properly. This could be cause of the tile not being installed correctly with the correct slope on the tile floor.

The microbial staining could also be due to residual organic glue used to face mount or back mount the glass tiles. This glue becomes a source of food for microbial growth.

Mold is a microbial growth that is ubiquitous, and as long is there isn’t a food source or a environment that promotes mold growth then it will not perpetuate.