Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large bowl, combine the softened cream cheese, squeezed chopped spinach, and garlic powder until smooth.
Stir the diced cooked chicken and sliced green onions into the spinach mixture and season with salt and black pepper to taste.
Stuff the chicken-spinach mixture into the cooked manicotti shells and arrange them in the prepared baking dish.
In a large saucepan over medium heat, melt the butter. Whisk in the flour and cook for about 2 minutes to form a roux.
Add the salt, black pepper, Italian seasoning, and crushed red pepper flakes to the roux, then gradually whisk in the milk until smooth.
Bring the sauce to a gentle simmer, whisking until it thickens slightly, then stir in the shredded Parmesan until melted and combined.
Pour the Parmesan sauce evenly over the stuffed manicotti, sprinkle a little extra Italian seasoning on top if desired, and bake for 30 to 35 minutes until bubbly and heated through.