In a 3 1/2- or 4-quart slow cooker combine hoisin sauce, the water, garlic, and crushed red pepper. Add mushrooms, stirring to coat.
Cover and cook on low-heat setting for 5 to 6 hours or on high-heat setting for 2 1/2 to 3 hours. To serve, remove mushrooms from cooker with a slotted spoon. Discard cooking liquid. Serve warm mushrooms with decorative toothpicks.