Make the Mango-Jicama 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.
Blend the Avocado Crema: In a small food processor or blender, combine the avocado, Greek yogurt, the remaining 2 tablespoons of cilantro, 1 tablespoon of lime juice, and the optional minced jalapeño. Add the ⅛ teaspoon of salt. Blend until smooth, adding the cool water 1 tablespoon at a time until you reach a smooth, drizzlable consistency.
Toast the Tortillas: Place the corn tortillas one at a time on a dry, hot griddle pan 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.
Season and Cook the Cod: In a small dish, stir together the ancho chili powder, smoked paprika, garlic powder, and the ¼ teaspoon of kosher salt. Evenly sprinkle the seasoning over all sides of the dry cod fillets. Heat the olive oil in the same pan over medium-high heat (about 425°F / 220°C). Carefully place the seasoned cod on the pan and cook for 3-4 minutes per side, until opaque and it flakes easily with a fork (internal temperature of 145°F / 63°C).
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.