Prepare the Sauce. In a medium saucepan over medium heat, melt the butter. Whisk in the flour and cook for 1 minute. Slowly add the milk, whisking constantly until smooth. Cook until the sauce thickens, then remove from heat. Stir in garlic powder, salt, and pepper.
Layer the Potatoes. Spray the inside of your slow cooker with nonstick spray. Layer half of the sliced potatoes in the bottom of the slow cooker. Pour half of the sauce over the potatoes, then sprinkle with half of the shredded cheese. Repeat with the remaining potatoes, sauce, and cheese.
Cook. Cover the slow cooker and cook on low for 6-7 hours or high for 3-4 hours, until the potatoes are tender and the cheese is melted and bubbly.
Bake (optional). Transfer the cooked scalloped potatoes into a baking dish. Place them in the oven at 400F for 10 minutes to allow the top layer to brown.
Garnish and Serve. Sprinkle fresh parsley over the top before serving. Enjoy these creamy, cheesy scalloped potatoes!