Squeeze 2 tablespoons juice from the remaining lemon into a large bowl. (Reserve the remaining juiced lemon for another use.) Add celery, scallions, mayonnaise, mustard, Old Bay and cayenne; stir to combine. Stir in shrimp. Cover and refrigerate until the salad is cold and the flavors have melded, at least 2 hours (and up to 3 days). Garnish with dill just before serving, if desired.