What caused this blue-green spot on field tile in shower?
We had this tile installed about 11 months ago. A spot developed - seemingly overnight. This is a Grazia Rixi field tile installed between a piece a Caesarstone used as the shower threshold/sill and the marble floor. The orientation in the photo is correct - the spot is coming from the right hand side of the tile. I wonder whether the copper pan has somehow leached onto the tile... I have noticed a few of the marble tiles on the floor develop a blue-green shade. The tiles were sealed.
Any ideas what could have caused this? Thanks!