1. Add the stock, fish sauce, garlic, chili pepper, lemongrass lemon and green onions to the Slow Cooker. Cover and cook on HIGH for at least 2 hours.
2. Add the frozen mussels and coconut milk. Continue to cook on HIGH, covered, for 15 to 20 minutes until the mussels are warm.
3. Stir in the cilantro and lime zest and juice. Add more fish sauce if desired.