Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally until cooked through but firm to the bite, 8 minutes. Drain.
Melt 1/4 cup butter in a large saucepan over medium heat; whisk flour, brown sugar, mustard, salt, and black pepper into butter. Pour in milk and bring to a simmer; reduce heat to low and whisk until sauce is thick and bubbly, about 5 minutes. Stir macaroni, ham, and apple slices into sauce. Spread mixture into a 2-quart casserole.
Combine soft bread crumbs with 2 tablespoons melted butter in a bowl; sprinkle crumbs over casserole.
Bake in the preheated oven until casserole is bubbly and crumb topping is browned, about 35 minutes.