Place the macaroni, butter, garlic powder, salt, black pepper, and water in the inner pot.
Stir. Make sure nothing is stuck to the bottom of the pot and scrape the bottom of the pot well.
Secure the lid on the pot and close the pressure-release valve. Select manual/high pressure for 5 minutes.
When the cooking is complete, use quick release to depressurize. However, release the pressure gradually and not abruptly or else the liquid will splutter. Use a towel.
Open the lid.
Select sauté, add milk, cheddar, and mozzarella cheese. You can add mayonnaise for an extra creamy texture (optional). Melt the cheeses by stirring for 30 seconds to 1 minute. The extra water will evaporate by the time both the cheese melt. Press cancel.
Transfer in a serving bowl immediately.