Preheat the oven to 400°F. Line a baking sheet with parchment paper.
In a mixing bowl, combine the ground turkey, ½ cup of the mozzarella, breadcrumbs, egg, garlic powder, and salt. Mix gently until the ingredients are evenly distributed.
Shape the mixture into 16 meatballs and place them on the prepared baking sheet. Bake for 15 minutes.
While the meatballs bake, warm the marinara sauce in a large skillet over medium heat.
Add the baked meatballs to the skillet and simmer for 5 minutes so they absorb some of the sauce.
Split the sub rolls and place 4 meatballs in each roll. Spoon extra sauce over the meatballs and sprinkle the remaining mozzarella on top.
Place the assembled subs on the baking sheet and return them to the oven for 3 to 4 minutes, just until the cheese melts.