Preheat oven to 375°F (190°C). Boil shells in salted water until al dente, 8–10 minutes. Drain and cool slightly.
In a bowl, combine shrimp, crab, ricotta, mozzarella, Parmesan, egg, parsley, garlic, salt, pepper, and red pepper flakes.
Melt butter in a saucepan over medium heat. Sauté garlic for 1 minute.
Add cream, simmer, then stir in Parmesan. Cook 3–4 minutes until thickened. Season and remove from heat.
Spread ½ cup Alfredo sauce in a 9×13 dish. Stuff each shell with seafood filling and place in the dish.
Pour remaining sauce over shells. Top with mozzarella. Cover and bake for 20 minutes, then uncover and bake 10 more until golden and bubbly.
Rest 5 minutes. Garnish with parsley and serve warm.