Combine the ground turkey, breadcrumbs, egg, minced garlic, Italian seasoning, salt, and black pepper in a mixing bowl. Stir until evenly mixed, then shape the mixture into 16 small meatballs.
Pour half the marinara sauce into the slow cooker. Place the meatballs in a single layer over the sauce, then cover them with the remaining sauce.
Cover and cook on low for 6 hours or on high for 3 hours until the meatballs reach an internal temperature of 165°F.
While the meatballs cook, use a spiralizer to turn the zucchini into noodles. Set the noodles aside in a bowl.
Spoon the meatballs and sauce over the zucchini noodles. Top each serving with Parmesan cheese and torn basil leaves.