Preheat a grill to high. Grill the corn, turning, until charred all over, about 8 minutes; let cool slightly, then cut off the kernels.
Toss the cucumber and bell pepper with 2 teaspoons salt in a colander. Let drain 30 minutes, then pat dry with paper towels.
Bring the vinegar, sugar, mustard seeds, turmeric and 1 tablespoon salt to a boil in a large skillet. Reduce the heat and simmer until the liquid is reduced to about 1 cup, 8 to 10 minutes. Add the cucumber and bell pepper; simmer until slightly softened, 3 minutes. Stir in the corn and scallions; let cool. Refrigerate until ready to serve.