Preheat oven to 350°F. Squeeze excess liquid from daikon and cucumber. Mix with yogurt, lemon juice, mint, and cayenne in a small bowl; season with salt and pepper. Set the raita aside. Heat 1 tablespoon oil in a large heavy pot over medium heat. Add garlic and cook, stirring constantly, just until fragrant, about 1 minute. Add collard greens to pot, reduce heat to low, cover, and cook, tossing occasionally, until tender, 10-15 minutes. Meanwhile, heat 1 tablespoon oil in a large ovenproof skillet over medium heat. Season fish with salt and pepper; cook skin side down until skin is crisp, 5-8 minutes. Transfer to oven (do not turn fish); roast until opaque in the center, about 4 minutes. Add radishes and vinegar to collard greens; season with salt and pepper and toss to combine. Divide greens, salmon, and reserved raita among plates.