Coat a 5-quart slow cooker with cooking spray. In a large bowl, stir together the sweet potatoes, pecans, butter and orange juice concentrate until combined. In a small bowl, stir together the brown sugar, pumpkin pie spice and cayenne pepper. Sprinkle over the sweet potato mixture and stir until combined. Spoon into the slow cooker.
Cover and cook on low for 2 to 3 hours.
Sprinkle the marshmallows over the top of the sweet potatoes in the slow cooker. Cover and cook on low for 15 more minutes, or until the marshmallows are melted.