Bring a large pot of salted water to a boil. Add the fettuccine pasta and cook according to the package instructions until al dente, typically about 10-12 minutes. Drain the pasta and set aside, reserving about 1/2 cup of the pasta water for later.
While the pasta cooks, heat the olive oil in a large skillet over medium-high heat. Toss the shrimp with Cajun seasoning, salt, and pepper. Add the shrimp to the skillet and cook for 2-3 minutes on each side until pink and opaque. Remove the shrimp from the pan and set aside.
In the same skillet, reduce the heat to medium and add the minced garlic. Sauté for about 30 seconds until fragrant but not browned. Pour in the heavy cream and bring to a gentle simmer, stirring frequently.
Gradually whisk in the grated Parmesan cheese until the sauce becomes smooth and creamy. If the sauce is too thick, add reserved pasta water a tablespoon at a time until you reach your desired consistency.
Add the cooked fettuccine and sautéed shrimp back into the skillet with the Alfredo sauce. Toss everything gently to coat the pasta and shrimp evenly with the sauce.
Sprinkle sliced green onions and fresh parsley over the top before serving. Serve immediately for the best texture and flavor.