Combine oats, milk, and salt in a 2½-quart slow cooker. Cover and cook at low heat for 6 to 8 hours. Before serving, melt butter in a small skillet over medium heat. Add the brown sugar and pineapple and cook just until pineapple starts to release its juices, about 2 minutes. Off heat, fold in the banana. Spoon topping and sprinkle with pecans.