Coat a 6-quart or larger slow cooker with cooking spray. Add the ziti, marinara, ricotta, and 1 cup of the mozzarella cheese, and stir well to combine. Sprinkle with the remaining 1 cup of mozzarella.
Cover and cook until the pasta is cooked through and the cheese is melted, on the LOW setting for 3 to 4 hours or on the HIGH setting for 1 to 2 hours. Serve immediately.