Cut the mozzarella cheese into ¾ inch cubes. In a large bowl, using your hands combine ground beef, Italian sausage, garlic powder, salt, black pepper, breadcrumbs, parmesan cheese, eggs, milk, and parsley.
Roll the meat mixture into golf ball sized balls and place a cube of the mozzarella cheese into the center and pull the edges of the meatball around it until it’s a ball again. Roll with your hands to seal tightly.
Pour a thin layer of the pasta sauce into the bottom of the slow cooker. Place a layer of the meatballs on top of the sauce. Pour the remaining sauce over the meatballs. Cover and cook on high for 2 to 2 ½ hours or until fully cooked (the internal temperature has reached 160 degrees F).