Lightly coat a 3 1/2- or 4-quart slow cooker with cooking spray. In a large bowl combine green beans, sweet pepper, Alfredo sauce, onion, water chestnuts, and garlic salt. Spoon bean mixture into prepared slow cooker.
Cover and cook on high-heat setting for 2 1/2 to 3 hours or on low-heat setting for 5 to 6 hours. Serve with slotted spoon. Sprinkle each serving with croutons (optional).