Grease slow cooker bowl with cooking spray and add sweet potatoes.
In a medium bowl, combine butter, sugar, cinnamon, vanilla, nutmeg if using, and a pinch of salt. Whisk to combine. Pour over sweet potatoes and toss until they are fully coated. Cook on high for 4 hours, stirring every hour.
Remove lid and top with marshmallows and pecans. Cover and cook on high about 10 minutes more or until marshmallows are melted.
