I was thinking something similar to Rafi89's reply just a bit ago:
If you can get to the plumbing from the other side of the wall, it could be what you need to do. Go to Youtube and look up Vancouver Carpenter on how to cut, and then patch a hole in drywall. He's got lots of great drywall vids. Once you've learned a little about how to do it RIGHT, it isn't so hard. But it's easy to do bad drywall jobs. I was doing bad ones for years until I found his vids.

Maybe you DON'T have to do this, and you can find cartridges that will replace what you've got. If you don't see the right thing at Home Depot or Lowes, find a plumbing supply, and pay a little premium for the free advice they've given to help you find - or not find a replacement.

However, I also do NOT like the looks of that grout. I suspect if you took some of that tile off you'd find black mold from moisture underneath. If you started taking tile off, you'd quite possibly end up putting up complete new concrete board to replace the sheetrock and doing new tiling. I wish you luck, and hope that regrouting does what you need!