Boil large pot of water; add tablespoon salt. Cook macaroni according to package until just tender. Reserve 1/2 cup pasta water; drain.
Meanwhile, melt butter in large saucepan over medium. Whisk flour 1 minute without browning.
Gradually whisk in milk; add mustard, sauce salt, and pepper. Gently simmer 4 to 5 minutes, stirring, until lightly spoon-coating.
Remove from heat; stir in cheddar and Monterey Jack by handfuls until smooth. Do not boil after adding cheese.
Fold in macaroni. Add reserved water 1 tablespoon at a time only as needed for flowing sauce. Serve promptly in warm bowl.