In a 3-qt. microwave-safe bowl, combine the cereals, crackers and pretzels. In a small microwave-safe bowl, melt butter. Stir in the vinegar, Worcestershire sauce, garlic powder, seafood seasoning, celery seed and mustard. Pour over cereal mixture and toss to coat.
Cook, uncovered, on high for 3-4 minutes, stirring once. Spread onto waxed paper to cool. Store in an airtight container.