Cook the pasta in boiling salted water to al dente, about 6-7 minutes, then drain and set aside.

In a medium saucepan, heat the butter and cheese on medium low heat and stir until completely melted.

Lightly spray a slow cooker with nonstick cooking spray and add the melted cheese and butter mixture, cooked noodles, milk, evaporated milk, mustard, salt and pepper. Stir to combine and cook covered on low for 2-2½ hours, stirring occasionally, until the noodles are tender and the sauce has thickened.

Add the Greek yogurt and stir until completely combined. Turn off the slow cooker and serve hot.