Combine the milk and macaroni in a medium saucepan. Bring to a boil over medium heat, stirring frequently to keep the macaroni from clumping, then cook, stirring frequently, until the macaroni is tender and the milk has thickened to the consistency of heavy cream, 4 to 5 minutes.
Remove the saucepan from the heat, add the Cheddar, mozzarella, cream cheese, butter, mustard, cayenne and 1 1/2 teaspoons salt and stir until smooth, thick and creamy. Stir in half the pulled pork. (The dish will thicken as it cools; thin it out with a little hot water if desired.) Spoon the remaining half of the pork over the mac and cheese and sprinkle with additional Cheddar and scallion.
Serve hot with barbecue sauce or hot sauce if using.
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