Preheat oven to 350 degrees F (175 degrees C). Spray a 2 1/2-quart baking dish with cooking spray.
Heat olive oil in a large skillet over medium heat; cook and stir eggplant, onion, and garlic until softened, about 8 minutes. Stir in diced tomatoes, zucchini, yellow squash, mushrooms, green bell pepper, and Italian seasoning; season with salt and black pepper. Bring the mixture to a boil and reduce heat to low; simmer until vegetables are tender, about 20 minutes.
Fill a large pot with lightly salted water and bring to a rolling boil. Stir in fettuccine, bring back to a boil, and cook pasta over medium heat until cooked through but still firm to the bite, about 8 minutes. Drain.
Mix Alfredo sauce with pumpkin in a bowl until smooth. Spread fettuccine into the prepared baking dish in an even layer, and spoon Alfredo sauce mixture over pasta. Top pasta with cooked vegetables, and sprinkle mozzarella cheese over the top.
Bake in the preheated oven until the casserole is bubbling and the cheese has melted, about 20 minutes.
