Add the dried mezzi rigatoni to the slow cooker and stir, making sure all of the pasta is submerged in the saucy liquid. It will look very loose at this stage—that’s what you want so the pasta can absorb enough moisture and end up fork-tender and swimming in sauce.
Sprinkle half of the shredded mozzarella (1/2 cup) over the top of the pasta mixture, but do not stir it in. This melts down as it cooks and helps create that rich, slightly cheesy, clingy sauce without turning into a solid cheese layer.