Place the spiral-sliced ham in a 6-quart or larger slow cooker, flat-side down.
In a small bowl, whisk together the brown sugar, maple syrup, pineapple juice, mustard, cinnamon, cloves, and garlic powder until a thick paste forms.
Rub the brown sugar mixture over the top and sides of the ham, pulling the slices apart slightly to get the glaze in between.
Cover the slow cooker tightly. If the ham is too tall, use a double layer of foil to create a dome before placing the lid on top.
Cook on the LOW setting for 3 to 4 hours, or until the ham reaches an internal temperature of 140°F.
Every hour, use a spoon to baste the ham with the sugary juices collecting at the bottom.
Once finished, carefully remove the ham and place it on a serving platter.
Optional: Transfer the remaining liquid to a saucepan and simmer over medium-high heat for 8-10 minutes until it thickens into a heavy syrup to pour over the meat.
Let the ham rest for 10 minutes before serving.