Bring a large pot of water to boiling. Cook scallops, shrimp, mussels, and calamari in boiling water for 2 minutes. Drain. Peel shrimp and shell scallops and mussels.
Place cooked seafood and olives in a large bowl and toss with lemon juice, olive oil, garlic, parsley, chives, and red pepper flakes. Chill for 1 hour.
Divide salad greens onto 6 plates or salad bowls. Spoon seafood over greens. Garnish with slices of lemon and red onions. Season with salt and freshly ground black pepper.