shower daily?
you have shower water coming down a bad wall. Grout is missing in many places. Shower water trickles in behind the tiles. It can seep through bad grout that only looks solid, too.
If you had only taken a weekly shower then the humidity might have dried out between uses, and the problem would not have gotten this far. Moisture is now deep inside that wall.
Molo, you can't let mold grow. It seems too late now for any "quick fix". The tiled wall has to be re-done. Rip it all out, put a vapor barrier this time, and tile on that. If you only redo the grout and silicone, you'll still have living mold -- billions of cells -- in that wall, grabbing whatever moisture they can get from the atmosphere, and continuing to offgas their toxins. Search on mycotoxins and see how bad they are. Mold, once grown, cannot be starved to death by dryness; it just doesn't work. The mold stays there and may not grow much more, but isn't killed by a new application of good grout and sealant.
Hope this is good news to you.
david
edit: i should have said that your tiles are butted together, so no grout will ever have any place to grip. The little grout that one might get to stick on the crack will cling to the glazing. Chunks of it will inevitably fall off.
- Your picture only shows the part of the wall that your camera could get visual access to. If you have a soap dish in the wall, there will tons of mold behind and around it.