Melt the butter in a medium-sized saucepan over medium-low heat. Add the celery and onion and cook, stirring occasionally, until the onions are translucent.
Add the flour, stir to incorporate, and cook for about 2 minutes or until fragrant. Add the milk and stir constantly until thickened. Add the cubed American cheese and stir until melted. Then add the crab and shrimp.
Pour the mixture into a greased 1 1/2–quart casserole dish and top with shredded cheese. Bake for 20 minutes and serve over hot cooked rice.