Mix the macaroni, water, mustard, and salt in your Instant Pot.
Close and lock the lid of the Instant Pot. Press “Manual” and adjust the timer to 4 minutes (or half the time on the macaroni cooking directions). Check that the cooking pressure is on “high” and that the release valve is set to “Sealing”.
When time is up, open the Instant Pot using “Quick Pressure Release”.
Stir the pasta to break it up. Add the evaporated milk, cheese, butter and nutmeg; stir until completely incorporated the and cheese has melted and coated the pasta.
Season to taste with salt and pepper, then serve immediately.