Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add cauliflower, cover, and steam until tender, about 25 minutes.
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 and transfer to a 9x13-inch casserole dish.
Blend cauliflower, yogurt, chicken broth, and olive oil in a blender until smooth; pour cauliflower mixture into a saucepan over medium heat. Stir Cheddar cheese, nutritional yeast, mustard powder, garlic powder, cayenne pepper, and nutmeg into cauliflower mixture; cook and stir until cheese sauce is smooth, about 5 minutes. Pour cheese sauce over macaroni; toss to coat.
Bake in the preheated oven for 15 minutes. Mix Parmesan cheese and panko crumbs together in a bowl; sprinkle over macaroni. Bake until Parmesan cheese is melted, 3 to 5 minutes.