Rinse the quinoa under cold water. Add it to a small saucepan with the water and bring to a boil. Cover, reduce the heat to low, and simmer until the water is absorbed and the quinoa is tender. Let it cool.
Heat the olive oil in a skillet over medium heat. Add the chopped spinach and cook, stirring, until wilted. Transfer it to a plate and let it cool.
Heat the oven to 375°F. In a large bowl, combine the ground turkey, cooled quinoa, spinach, egg, Parmesan, Italian seasoning, garlic powder, salt, and black pepper. Mix gently until evenly combined, taking care not to overwork the turkey.
Shape the mixture into a loaf in a baking dish. Spoon ½ cup of the marinara over the top, spreading it into an even layer.
Bake until the center reaches 165°F on an instant-read thermometer. Sprinkle the mozzarella over the loaf and add the remaining marinara. Return it to the oven until the cheese melts.
Let the meatloaf rest for several minutes before slicing. This helps the slices hold together.