Preheat the oven to 350 degrees F (175 degrees C).
Bring 2 large pots of lightly salted water to a boil. Cook elbow macaroni in the first pot, stirring occasionally, until tender yet firm to the bite, about 8 minutes. Cook cauliflower in the second pot, stirring occasionally, until tender yet firm to the bite, 3 to 6 minutes. Drain macaroni and cauliflower.
Return 1 empty pot to the stove. Add olive oil and heat over medium heat. Add garlic to the hot oil and saute until fragrant, about 1 minute. Stir in red pepper flakes. Add drained cauliflower, reduce heat to medium-low, and saute for 2 minutes.
Add the cooked macaroni to the cauliflower mixture. Add 1 3/4 cups each mozzarella and Cheddar cheeses and stir until all ingredients are well combined. Transfer to a baking dish and sprinkle with remaining mozzarella and Cheddar.
Bake in the preheated oven until nice and bubbly, about 30 minutes.