Combine the macaroni, butter, water, mustard and pepper in the Instant Pot. Secure the lid, making sure the vent is closed.
Using the display panel select the MANUAL function*. Use the +/- keys and program the Instant Pot for 4 minutes.
When the time is up, quick-release the pressure. If pasta is not completely cooked, use the SAUTÉ function and cook, stirring occasionally, for another 2-3 minutes.
Add evaporated milk and cheeses, one at a time, stirring between each until the mixture becomes thick and creamy.
Salt to taste.
Notes
*The MANUAL and PRESSURE COOK buttons are interchangeable.