Spray slow cooker with non stick spray.
Peel and dice sweet potatoes.
Place potatoes in slow cooker.
Combine brown sugar, butter, pumpkin pie spice, vanilla and water in microwavable bowl.
Microwave for 30 seconds and stir. Repeat until mixture is well blended and warmed.
pour mixture over sweet potatoes and mix to distribute.
Cover and cook on high for 4 hour or until tender.
Distribute marshmallows over top and then distribute pecans over marshmallows.
Reduce heat to low and recover until melted.