Grease the slow cooker with cooking spray.
Peel and chop sweet potatoes into medium-sized cubes and place them in the Crockpot.
In a mixing bowl, whisk melted butter, brown sugar, cinnamon, vanilla extract, nutmeg (if using), and a pinch of salt.
Pour the mixture over the sweet potatoes and toss to coat evenly.
Cover and cook on low for 3-4 hours, stirring once halfway through.
Sprinkle marshmallows and pecans on top, then cover for 5-10 minutes until marshmallows melt.
Serve warm and enjoy this cozy, caramelized side dish!