Add butter to the pan and reduce heat to medium. Add garlic and crushed red pepper; cook, stirring often, until the garlic is fragrant, about 1 minute. Add arugula and cook, stirring, until wilted, about 1 minute. Reduce heat to low. Add the fettuccine, yogurt, lemon zest and the reserved cooking water, 1/4 cup at a time, tossing well, until the fettuccine is fully coated and creamy. Add the shrimp, lemon juice and salt, tossing to coat the fettuccine. Remove from the heat and toss with Parmesan.