Cook macaroni in a large pot of water with the salt until just tender. Reserve 1/2 cup pasta water and drain.
Meanwhile, cook bacon in a large skillet over medium heat for 8 to 10 minutes until crisp. Transfer to a plate and leave exactly 2 tablespoons rendered fat in the skillet, discarding excess.
Whisk flour into the fat for 1 minute, then gradually whisk in milk. Add mustard and pepper and simmer for 4 minutes until lightly thickened.
Remove from the heat and stir in white cheddar until smooth. Fold in pasta and half the bacon, loosening with reserved pasta water a tablespoon at a time if necessary.
Scatter the remaining bacon over the pasta and serve promptly.