Slice lemon grass stalk thinly crosswise. Peel and halve cucumbers lengthwise, then seed and slice thin crosswise.
Relish: Bring lemon grass, vinegar, sugar, red pepper flakes and 1/2 cup water to boil in a saucepan; simmer until reduced to 1/2 cup. Strain mixture into a bowl and cool slightly; discard solids. Stir in all other sauce ingredients.
Sprinkle scallops with 1/2 teaspoon salt and 1/4 teaspoon white pepper. Heat one tablespoon oil in a wok or non-stick skillet. Working in batches to avoid overcrowding, and adding remaining oil as needed, saute scallops, turning once, until seared.
Spoon a portion of relish onto each plate. Arrange a portion of scallops over the relish and serve immediately.