Make the salsa: In a large bowl, combine the peaches, cucumber, red onion, jalapeno, cilantro and lime juice and season with salt and pepper. Cover and refrigerate until ready to serve.
Make the fish: Season the mahi mahi with salt and pepper. Heat the olive oil in a nonstick skillet over high heat. Sear the fish until opaque in the center, 3 to 4 minutes per side, depending on the thickness. Transfer the fish to a platter and top with some of the salsa. Serve any extra salsa on the side.