Whisk together the milk, egg whites, and cornstarch in your slow cooker insert. Be sure to whisk well.
Stir in the grated cheese, broccoli, and pasta shells.
Cook on low for 1-1/2 to 2 hours.
After the first hour, stir the food on a semi-regular basis (about every 15 minutes or so). This will do two things. It will allow the pasta to cook evenly. It will also allow you to keep any eye on the pasta and see when it's done.
Every slow cooker is different, so the timing for this recipe may be slightly different on this for you. Pasta goes from "cooked" to "mush" very quickly. So be sure to keep stirring on occasion to keep an eye on things.