Cut away peel and pith from grapefruit. Cut along membranes, releasing segments into a medium nonreactive bowl; reserve juice in bowl. Using a slotted spoon, transfer 1/2 of the segments to a cutting board; dice. (Reserve remaining segments for another use.) Transfer diced segments to a large nonreactive bowl. Strain reserved juice into a glass measuring cup. Pour 1/4 cup of the juice into the large bowl, reserving remaining juice for another use.
Add onion, chile, mint, lime juice, salt, and scallops or yellowtail to grapefruit mixture. Gently stir until well combined. Cover with plastic wrap, and refrigerate for 2 hours. (Do not refrigerate for more than 2 hours; the scallops and yellowtail will become tough.)
Drain mixture, reserving liquid. Divide onion, chile, mint, and scallops or yellowtail among 4 serving plates. Garnish with micro mint, and drizzle with reserved liquid. Serve immediately.
Any variety of mint will work, but the citrus aroma of grapefruit mint pairs perfectly with the scallops and grapefruit. Micro mint, which are tiny flavorful sprouts, are ideal for the garnish. Sea scallops should have a firm texture, a pearly, almost peach-colored tone, and a faintly sweet aroma. Ask for scallops that are dry or dry-packed; these have a fresher flavor and richer texture than those packed in a saline solution.