Judging by the efflorescence on part of the wall, it looks like you already have water issues behind the stone. I’d strongly suggest not putting a water feature on the wall, as it has not been designed for it. You will need lots of waterproofing behind the stone, as well as additional depth for the plenum tank to distribute the water for even flow down the face. If you want a waterfall, I’d try to find a similar stone and building one on a freestanding wall in an area where you can see it from inside the house. You are asking for trouble building it into an existing structure not originally designed for it.