Preheat oven to 400 degrees F (200 degrees C). Rinse the fillets and pat dry with paper towels.
Combine crab meat, green pepper, mustard powder, Worcestershire sauce, salt, white pepper and the crushed saltines. Combine the egg white and 1 tablespoon mayonnaise. Stir this into the crab meat mixture.
Brush the flounder fillets with melted butter. Place in a lightly greased, shallow baking dish. Spoon the crab mixture over the fillets and drizzle with any remaining butter.
Bake the fillets at 400 degrees for 15 minutes.
While the fish is baking, lightly beat the egg yolk in a small bowl. Stir in 5 tablespoons of mayonnaise. Remove fish from oven and spread this mixture over the stuffing; sprinkle with paprika and parsley.
Increase oven temperature to 450 degrees and bake until golden and bubbly, about 6 minutes.