Combine soup and cream cheese in top of a double boiler; cook over hot water until cream cheese melts and is well blended. Remove from heat; cool slightly. Stir in gelatin; mix well. Add shrimp, mayonnaise, celery, green onion, green pepper, Worcestershire sauce, and lemon juice; mix well.
Pour mixture into a lightly oiled 5 1/2-cup fish mold. Cover and chill overnight.
Unmold mousse onto a chilled serving plate. Decorate fish with pimiento-stuffed olive slices for eyes and mouth, and cucumber slices for gills and tail, if desired. Garnish tray with parsley, if desired. Serve immediately with crackers.