For the Spicy Mayo, mix all ingredients in small bowl until well blended. Cover and refrigerate until ready to serve.
For the Mahi Mahi Tacos, mix cumin, smoked paprika, black pepper and salt in small bowl. Season fish evenly with lime juice and seasoning mixture. Grill over high heat 3 to 5 minutes per side or until fish flakes easily with a fork. (Mahi mahi cooks quickly, so try not to overcook it.) Transfer to cutting board. Cut each fillet into 4 strips.
To serve, place 1 fish strip on warm tortilla. Top with desired amount of Spicy Mayo, salsa, avocado slices and green onions. Serve tacos with lime wedges, if desired. (Store any leftover Spicy Mayo in covered airtight container in refrigerator.)