Bring a large pot of lightly salted water to a boil. Add green beans and cook uncovered until bright green, about 3 minutes. Drain in a colander and immediately immerse in ice water for several minutes to stop the cooking process. Drain and transfer to a large bowl.
Mix mustard powder and cold water together in a small bowl to make a paste. Stir in vinegar, soy sauce, sesame oil, sesame seeds, sugar, and salt.
Pour mustard paste mixture over green beans; toss to coat. Chill for at least 30 minutes before serving.
1 pound fresh green beans, trimmed
2 teaspoons dry mustard powder (such as Coleman's®)