Toss thawed corn, rinsed black beans, and diced red pepper with juice of half the lime, 1 tablespoon olive oil, and chopped cilantro in a bowl, then set aside.
Pat mahi-mahi dry, coat both sides generously with Cajun seasoning, and heat 2 tablespoons olive oil in a large skillet over high heat.
Cook mahi-mahi for 4 minutes per side until the spice crust is dark and fish flakes easily, then transfer to a plate.
Serve fish alongside the corn and black bean salad, finishing with lime wedges.