Preheat the oven to 400°F Cut the bread into 2 triangles for jack o’ lantern eyes, 1 large mouth, and 1 stem.
Bring a large pot of generously salted water to a boil. Add the cauliflower and cook until tender, about 5 minutes. Use a slotted spoon to transfer the cauliflower to a blender. Add the macaroni to the boiling water and cook until just tender, about 4 minutes. Drain and transfer to a 9-inch pie dish.
While the macaroni cooks, add the cream, mustard, and 1 1/2 cups cheese to the blender. Puree until smooth. Season to taste with salt and pepper. Pour over the macaroni and gently stir until well-combined. Top with the remaining 1/2 cup cheese, then the pumpernickel croutons.