Prepare the Slaw: In a medium bowl, combine the jicama matchsticks, diced mango, thinly sliced red onion, 2 tablespoons of chopped cilantro, and 1 tablespoon of lime juice. Toss gently to combine and set aside for the flavors to meld.
Make the Avocado Crema: In a small food processor or blender, combine the avocado, non-fat Greek yogurt, the remaining 2 tablespoons of cilantro, 1 tablespoon of lime juice, and the optional minced jalapeño. Blend until completely smooth, adding the cool water 1 tablespoon at a time until you reach a smooth, drizzlable consistency.
Warm the Tortillas: Place the corn tortillas on a dry, hot griddle or skillet over medium-high heat. Warm for about 45 seconds per side, until pliable and lightly browned. Stack the warmed tortillas and wrap them in a clean kitchen towel to keep warm.
Cook the Cod: In a small dish, stir together the ancho chili powder, smoked paprika, garlic powder, and onion powder. Evenly sprinkle the salt-free seasoning over all sides of the dry cod fillets. Heat the olive oil in the same skillet over medium-high heat. Carefully place the seasoned cod in the pan and cook for 3-4 minutes per side, until opaque and it flakes easily with a fork.
Assemble and Serve: Transfer the cooked cod to a plate and use a fork to break it into large chunks. To build your tacos, place a generous amount of the slaw onto each warm tortilla. Top with the flaked cod and finish with a drizzle of the avocado crema. Serve immediately with fresh lime wedges on the side.