Good point; read the can to see if impermeable or not... would high-pressure water remove grout?
It don't think it can remove anything that isn't loose. Mortar joints should be safe. ( Cmu is set with mortar, filled with grout. I often find myself saying mortar when I mean grout, and vice versa. They are similar.)
Here's a photo of cmu being water blasted. My first thought was light sandblasting, which is also an option.