Cook the pasta: Bring a large pot of water to a boil. Cook the pasta according to the package directions. Drain and let it cool completely.
Cook the shrimp: Put the shrimp in a large bowl, season with ¼ teaspoon of the Old Bay and the garlic powder, and toss to coat all over.
In a large skillet, melt the butter over medium heat. When the butter is melted, place the shrimp in the pan in a single layer and cook for 2 to 3 minutes, until they are pink and opaque. Flip and continue cooking for another 1 to 2 minutes, until they're pink all over and just cooked through. Transfer the shrimp to a plate and set aside to cool completely.
Make the salad: In a large bowl, whisk together the mayonnaise, mustard, relish, lemon juice, and Worcestershire sauce. Add the celery, bell pepper, onion, pasta, and shrimp and toss until everything is well coated. Gently fold in the crab and season with the remaining 1 teaspoon Old Bay.
Chill: Cover the bowl and refrigerate the pasta salad until chilled, for at least 1 hour and up to overnight before serving.