What will happen if the AC unit is mounted on the brick wall?
If a structure was designed to hold a condenser this wouldn't be an issue. However, you stated that there was a concrete pad it used to sit on. If installed properly there would have to be access on the other side of the wall. Meaning, sheetrock would have to be removed and blocking installed. If they just attached a platform to the brick using a wedge anchor, epoxy, tapcons, or how ever they did it, I can't imagine it holding for long. Yes, those things vibrate, and yes eventually it will jiggle the bricks enough to break the grout. What kind of "brick" is it? Sneak over there and take a photo, cuz I think we ALL would love to see it.