Cook the fettuccine pasta according to package instructions until al dente. Drain and set aside, reserving 1/2 cup of pasta water.
In a large skillet, heat the olive oil over medium heat. Add the shrimp and cook for 2-3 minutes on each side until they turn pink and opaque. Remove the shrimp from the skillet and set aside.
In the same skillet, add the minced garlic and sauté for about 30 seconds until fragrant.
Stir in the Cajun seasoning, paprika, and cayenne pepper (if using). Cook for another minute to toast the spices.
Pour in the heavy cream and bring to a gentle simmer. Stir in the Parmesan cheese until melted and the sauce is smooth.
Add the cooked fettuccine, shrimp, and crab meat to the skillet. Toss everything together, adding reserved pasta water as needed to reach desired sauce consistency. Season with salt and pepper to taste.
Serve immediately, garnished with chopped parsley.