Place the macaroni in the Instant Pot. In a small bowl, combine the salt, pepper, ground mustard, garlic powder and green chile powder (if using). Sprinkle the seasonings over the dried noodles. Pour in water, stirring to combine. Add the butter. Place the lid on the Instant Pot, and adjust the vent knob to "sealing." Turn the pot on Manual at High Pressure, and set the timer to 6 minutes.

When the timer goes off, quick release the pressure by flipping the vent knob to "venting." Remove the lid and add the cubed cream cheese, stirring until cheese melts and coats the pasta. Add the shredded cheddar and monterey jack cheeses and the milk, stirring well until cheeses melt and all ingredients are fully incorporated. Serve immediately.