Cook Pasta Shells: Boil shells until al dente. Drain and set aside.
Prepare Filling: Mix shrimp, crab, ricotta, parmesan, egg, parsley, garlic, salt, and pepper in a bowl.
Stuff Shells: Spoon filling into each shell and place in baking dish.
Make Alfredo Sauce: Melt butter in saucepan, sauté garlic, add cream, and simmer until thickened. Stir in parmesan until smooth.
Assemble Dish: Pour Alfredo sauce over stuffed shells. Cover with foil.
Bake: Bake at 375°F (190°C) for 20 minutes covered, then uncover and bake 10 minutes until bubbly and golden.
Serve: Garnish with parsley and serve hot.
Expert Tip: Cover with foil during the first half of baking to prevent shells from drying out.