Cook the Pasta:
Bring a pot of salted water to a boil and cook macaroni just until al dente. Drain and set aside.
Mix the Sauce:
In a large bowl, whisk milk, evaporated milk, eggs, melted butter, salt, pepper, and dry mustard until fully combined.
Layer in the Crockpot:
Spray the inside of your crockpot with non-stick spray. Add half the macaroni, sprinkle half the cheese, repeat layers, and pour the milk mixture evenly on top.
Slow Cook:
Cover and cook on LOW for 2 ½ to 3 hours. Stir once halfway through to prevent sticking.
Finish and Serve:
When the cheese is melted and sauce creamy, turn off the crockpot. Let it rest for 10 minutes before serving.