In ungreased 2-quart slow cooker, mix flour and mustard. In small bowl, mix cider, Worcestershire sauce and pepper sauce with whisk until blended. Stir into flour mixture until blended. Stir in Cheddar cheese and American cheese.
Cover; cook on High heat setting 1 hour 30 minutes, stirring twice, until cheese is melted and mixture is smooth. Serve dip warm with pretzels and apples.