Combine the elbow macaroni, cauliflower florets, milk, broth, butter, garlic powder, salt, and pepper in a large pot.
Bring the mixture to a boil over high heat, then reduce to medium heat and cook for 12-14 minutes, stirring frequently, until the pasta and cauliflower are both tender.
Reduce heat to low and stir in the Dijon mustard, cheddar cheese, and Gruyere cheese, stirring constantly until all the cheese is melted and the sauce is creamy.
If the sauce seems too thick, add a splash of milk; if too thin, let it simmer for 1-2 more minutes to thicken.
Remove from heat and let stand for 3 minutes before serving to allow the sauce to set to perfect creaminess.