Heat olive oil in a large saucepan or dutch oven over medium heat.
Add shallots and garlic to saucepan and sauté, stirring until they just begin to color, about 2 to 3 minutes.
Add the grapefruit shandy to the pot and bring to a boil. Reduce to a simmer and add the mussels to the pot. Cover and let the mussels cook until the shells open, about 4 to 6 minutes.
Add 2 Tbsp butter and ½ Tbsp parsley to the pot and stir to combine.
With a slotted spoon, transfer mussels to separate bowl. Spoon (or pour) some of the cooking liquid over the mussels.
While the mussels are cooking, bring a pot of lightly salted water to a boil. Add linguine to pot and let pasta cook until al dente. Drain pasta and return to pot with remaining 2 Tbsp butter.
Add in grapefruit juice, grapefruit zest, and the remaining ½ Tbsp parsley.
Serve linguine in bowls and top with mussels and grated parmesan cheese. Add grapefruit wedges for garnish.