I believe it's water. Do you have a piece of this tile - to soak in water?

Defective glaze would be my guess. Moisture between the glaze & tile body.

geoffrey_b; cat_ky : thank you for your comments, but I'm not clear. The tiles are in the shower stall so they have water on them every day. I didn't install them; it's a 2yr old new build house in a subdivision. The tiles are from Olympia Tile in Toronto - I picked them and paid extra for them. When they built the house they used orange board behind shower. I forget the name of it, and sub-contractor tile for the builder was supposed to be very good. What specific type of waterproofing would the tiles require?

You can do a search on this forum for "Shower waterproofing" and read what the pros have to say when people are having trouble with their showers, and you will learn a lot. The search box is at the top of the page.

You may not understand how water gets in back of the tiles that are installed and grouted unless you know that grout is not waterproof. That is why the surface behind the tile must be waterproofed. Water should evaporate again, once the shower is not i use, or the shower system should have weep holes in the drain that allow any drips behind the tile to go down the drain. If there is not waterproofing, then the cement board (or improperly used drywall) behind the tile gets wet. If there is direct contact with wood, then it soaks up the moisture and voila! you have mold and rot.

If you have water proofing and Creative Tile Eastern CT is right and it is a glazing issue with your tile, it may be that you have good waterproofing behind it. The bad news is that removing the defective tile will likely ruin the waterproofing. So read up on waterproofing and understand how it is supposed to be done (there are several methods) so that you can discuss it with the person you hire to replace the shower. Many, many handymen and even bathroom contractors do not use approved methods. Reading those threads you search for here will help you with the education you are going to need to have this fixed right.

I might start by contacting the tile manufacturer and sending them photos and asking if they are having issues with their tile glazing. Pros here often recommend that you hire someone who is a member of the Tile Council of North America (TCNA) to assess the problem. Go read some threads and you will catch on to how it all works.

Yes, they have water on them every day, but, that does not mean they are water proof to whatever is behind them. Nancy above explained it pretty well. Please do a search here, and I am sure you will find a lot of info on proper waterproofing, as well, as some of the messes and problems, others have had.

You don't waterproof the tiles, you waterproof the surface they are adhered to. If the contractor only installed the orange board, but didn't do the rest of the steps to waterproof it, that's a problem.

Looks like moisture getting in to the body of the tile through the grout lines and through the edge of the tile body. Moisture ends up getting trapped behind the glaze.

There's typically not anything for mold to thrive on in that location, so it's typically just discoloration due to moisture.

Sometimes with repeated use of the shower the moisture will hold there. It may expand if wetting exceeds drying. Or dry via reversing the path the moisture entered the tile. Or it can dry through the back of the tile unless the tile was set on a surface applied membrane. Many variables.

I assume the 'Orange Board' is Kerdi Board. That 'IS' the waterproofing (as long as he taped the seams.) I see they are larger tiles, and glazed, and grouted. I believe it's a defect in the tile.

Thank you to everyone who commented. I am in touch with the supplier of the tiles as well as the builder. I may be out of luck because any warranty period has passed. Right now I'm not using the shower - just hoping it won't get worse, maybe reverse if the moisture dries. The tiles were sold as bathroom wall tiles, but apparently that doesn't necessarily mean they can be used in the wall of a shower. I should have been told that by the builder's designer if that was the case. Any way, I'm not in a position to rip it all out and replace it right now so I will wait and see what the builder says.