Cook the macaroni according to package directions. Drain.
While the macaroni is cooking, melt the butter in a small saucepan over low heat.
Using a wooden spoon, stir the flour, dry mustard, garlic powder, and black pepper into the melted butter.
Cook over low heat, stirring until the mixture is smooth and bubbly.
Stir in milk, about a quarter cup at a time, making sure that the milk is incorporated after each addition.
Once you have added all the milk, boil for about 1 minute, stirring constantly.
Stir the nutritional yeast, mozzarella, and cream cheese into the sauce.
Heat the mixture long enough for the cheese to melt. Remove from heat.
Stir the cooked macaroni into the sauce and serve immediately.