In a large bowl, combine the sliced cucumbers and salt. Toss well and let them sit for about 15 minutes to draw out excess moisture.
After 15 minutes, rinse the cucumbers under cold water and drain thoroughly. Pat them dry with a paper towel.
In a separate bowl, whisk together the rice vinegar, sugar, soy sauce, sesame oil, minced garlic, and red pepper flakes (if using) until the sugar is dissolved.
Add the drained cucumbers to the dressing and toss to coat evenly.
Sprinkle with sesame seeds and chopped green onion. Toss gently to combine.
For the best flavor, let the salad chill in the refrigerator for at least 30 minutes before serving.