Cook manicotti according to package directions; drain. Place manicotti in a single layer on a sheet of greased foil.
Meanwhile, preheat oven to 350 degrees F. In a medium bowl combine shrimp, crabmeat, 1 cup of the Italian cheese, the ricotta cheese, parsley, egg, garlic, and lemon peel. Using a small spoon, fill manicotti with shrimp mixture; arrange filled manicotti in an ungreased 13x9x2-inch baking dish. Pour Garlic Cream Sauce over filled manicotti. Sprinkle with the remaining 1/2 cup Italian cheese.
Bake, covered, for 25 minutes. Bake, uncovered, about 10 minutes more or until heated through and cheese is melted. If desired, garnish with green onions and lemon slices.
Garlic Cream Sauce
In a medium saucepan cook green onions and garlic in hot butter until tender and fragrant. Stir in the flour. Add milk all at once; cook and stir until thickened and bubbly. Stir in the ricotta cheese, black pepper, and salt.