To make the mango salsa, combine mango, cucumber, shallot, Serrano pepper, and cilantro in a bowl. Toss with lime juice and season with salt and pepper to taste. Mix well and set aside.
To prep the scallops, quickly rinse them under cold water. Check each scallop for its side muscle. To remove it, pinch and peel it off with your hands. Transfer the cleaned scallops to a plate lined with a paper towel. Gently press a second piece of paper towel over top to remove as much excess moisture as possible. Season both sides with salt and set aside.
Heat a large skillet over medium-high. Pour in oil to lightly coat the surface. Heat the oil until it shimmers and you see first wisps of smoke.
Working in batches to avoid overcrowding the pan, place the scallops into the skillet, flat side down, and cook without touching or tossing until the underside is deep golden brown, 2-3 minutes. Use a thin spatula to gently turn them over. Cook on the second side until the flesh at top and bottom looks opaque but there is still a faintly translucent strip in the middle, about 2 minutes, depending on size. Transfer scallops to a plate.
When ready to serve, spoon the mango salsa onto a plate and top with scallops.