Heat oil in a large Dutch oven over medium heat. Add mustard seeds; cook 2 minutes or until seeds begin to pop. Add onion and curry leaves; cook 8 minutes or until onion is lightly browned, stirring occasionally. Stir in juice, ginger, 1/4 teaspoon salt, 1/4 teaspoon pepper, garlic, and chile.
Sprinkle squid with remaining 1/4 teaspoon salt and remaining 1/4 teaspoon pepper. Add squid and mussels to pan; cover and cook 3 minutes or until mussels open and squid begins to curl around edges. Discard any unopened shells. Remove from heat, and sprinkle evenly with cilantro.