Stir in the shell pasta, and return to a boil. Cook uncovered, stirring occasionally, until the shells have cooked through, but are still firm to the bite, about 10 minutes. Spread the spinach over the top of the pasta, cover, and simmer until the spinach leaves are cooked, about 5 minutes. Sprinkle the mozzarella cheese evenly over the skillet, and simmer until the cheese has melted and the pasta is bubbling, about 5 minutes.