Preheat the oven to 425˚ F. Line a baking sheet with parchment paper; set aside. Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs for al dente. Reserve 1/2 cup cooking water, then drain. Reserve the pot.
Meanwhile, combine 1/2 cup crushed cheese crackers and 1/4 cup milk in a medium bowl; let sit until slightly softened, about 10 minutes. Add the sausage and 2 tablespoons Parmesan and mix well to combine. Form the sausage mixture into 1-inch meatballs (about 28); arrange on the prepared pan. Bake until golden and cooked through, 6 to 8 minutes.
Add the flour and the remaining 2 cups milk to the reserved pot and whisk until smooth. Bring to a simmer over medium-high heat and cook, whisking, until slightly thickened, 8 to 10 minutes. Remove from the heat, add the cheddar and processed cheese and stir until smooth. Stir in 3/4 teaspoon salt and the mustard.
Add the pasta and 1/4 cup of the reserved cooking water to the cheese sauce and stir to coat. Add the meatballs and stir to coat; thin with the remaining 1/4 cup cooking water, 1 tablespoon at a time, if needed.
Combine the remaining 1/2 cup crushed cheese crackers and 1 tablespoon Parmesan in a small bowl. Divide the mac and cheese among bowls and sprinkle with the cracker mixture.