Cook pasta according to package directions with ½ teaspoon of the salt. Cook to al dente. Drain and set aside.
In a large skillet, melt the butter over medium heat. Stir in the pumpkin, pasta water, nutmeg, remaining salt, pepper, and garlic powder.
Gradually stir in the milk. Add the lemon juice, reduce heat to low and simmer for 5 to 10 minutes. Stir in the Parmesan and spinach, stir frequently until cheese is melted.
Stir in the prepared pasta, remove from heat and serve with additional Parmesan.