Preheat your oven to 350°F (175°C).
In a saucepan, combine the cream cheese with 2 tablespoons of butter. Cook over low heat until melted and smooth, stirring constantly.
In a large ovenproof skillet, melt the remaining 2 tablespoons of butter over medium heat. Add the chopped onion, celery, and green pepper, sautéing until tender (about 5-7 minutes).
Stir in the cooked shrimp, flaked crabmeat, condensed cream of mushroom soup, cooked rice, drained mushrooms, garlic salt, hot pepper sauce, and cayenne pepper. Mix well to combine.
Pour the melted cream cheese mixture into the skillet and stir until everything is evenly coated.
In a small bowl, combine the shredded cheddar cheese and crushed crackers. Sprinkle this mixture over the casserole.
Bake the casserole uncovered for 25-30 minutes, or until the top is golden and bubbly. If desired, garnish with thinly sliced green onions before serving.