Heat grill to medium-high. Cook the rice according to the package directions.
Meanwhile, in a small bowl, stir together the hoisin and Sriracha; oil grill. Brush the steak with the hoisin mixture and grill, 2 to 3 minutes per side. Let rest for 5 minutes before slicing.
In a medium bowl, toss the cucumber and scallion whites with the vinegar, sugar, and ¼ teaspoon each salt and pepper. Serve with the steak and rice and sprinkle with the peanuts and scallion greens.