Prep the Slow Cooker: Line the base of a 5- to 7-quart slow cooker with a strip of aluminum foil or parchment paper to create a "sling" with handles extending up the sides. Lightly coat the inside and sling with nonstick cooking spray.
Combine Ingredients: In a large bowl, whisk together the egg, milk, olive oil, minced garlic, Italian seasoning, garlic powder, onion powder, salt, and black pepper. Stir in the breadcrumbs, Parmesan cheese, and ¼ cup of the shredded mozzarella. Let sit for 2 minutes to absorb the liquid.
Shape the Meatloaf: Add the ground chicken to the bowl. Gently mix with your hands or a fork until just combined—do not overmix. Transfer the mixture to the slow cooker and shape into a compact, oval loaf over the foil sling, leaving space around the edges for heat to circulate.
Slow Cook: Cover and cook on LOW for 3 to 4 hours (or HIGH for 1.5 to 2 hours) until an instant-read thermometer inserted into the thickest part of the loaf registers 165°F (74°C).
Melt Cheese & Rest: Top the meatloaf with the remaining ¼ cup mozzarella cheese. Cover and let sit on warm for 5 to 10 minutes until melted. Use the foil sling to carefully lift the meatloaf out onto a cutting board. Rest for 10 minutes before slicing to retain juices.