Preheat oven to 350 degrees F.
Bring saucepan of water to a boil. Cook macaroni for approximately 2 minutes less than the directions on the box; drain and set aside.
Meanwhile, put cauliflower in a microwave safe bowl that can be covered and microwave until lightly steamed, 2-3 minutes. Drain any excess liquid and finely chop.
In a small bowl, combine the breadcrumbs, parmesan cheese and olive oil.
In a large saucepan, whisk the milk and flour together until the flour is dissolved. Whisking constantly, bring to a gentle boil over medium heat, then reduce the heat to medium-low and simmer until mixture thickens slightly, 3-4 minutes.
Stir in the cheddar, gruyere, mustard powder, paprika, ½ teaspoon each salt and pepper, and cayenne. Whisk until the cheeses are melted and mixture is smooth, 1-2 minutes.
Add the macaroni and cauliflower and stir until well coated. Stir in crab.
Spray a 2 quart casserole dish with cooking spray and pour mixture into skillet. Sprinkle with the breadcrumb mixture, bake until top is browned and the cheese is bubbly, about 30 minutes.
