Pour alfredo sauce into a large saucepan set over medium heat. Meanwhile, slice crab into chunks. Stir in crab and nutmeg. If pasta is not ready yet, reduce heat to low and cover to keep warm.
When pasta is cooked, drain well. Then add to sauce in pan. Stir in spinach. If sauce is too thick, stir in milk, 1/4 cup at a time. Stir until warmed through, 1 or 2 more minutes. Serve in big pasta bowls.