In a large bowl, combine the sliced cucumbers and salt. Toss to coat and let sit for 15 minutes to draw out excess moisture.
After 15 minutes, rinse the cucumbers under cold water and drain well. Pat dry with a paper towel.
In a separate bowl, whisk together the sugar, rice vinegar, soy sauce, sesame oil, minced garlic, and red pepper flakes (if using).
Add the drained cucumbers to the dressing and toss until well coated.
Stir in the sesame seeds and chopped green onion.
Let the salad sit for at least 10 minutes before serving to allow the flavors to meld. Serve chilled or at room temperature.