Preheat oven to 375°F (190°C) and lightly grease a 9x13 inch baking dish.
Boil penne pasta in a large pot of salted water until al dente, according to package directions; drain.
In a large mixing bowl, whisk together the ranch dressing, milk, garlic powder, and black pepper.
Stir the cooked pasta, cubed chicken, and 1 cup of shredded cheddar cheese into the ranch mixture until thoroughly combined.
Transfer the pasta mixture into the prepared baking dish, spreading it into an even layer.
Top evenly with the remaining 1 cup of cheddar cheese, the mozzarella cheese, and the crumbled bacon.
Bake for 20 to 25 minutes until the cheese is melted, bubbling, and slightly golden around the edges.
Remove from oven and garnish with sliced green onions before serving.