Thinly slice the cucumbers and place them in a large mixing bowl. If you prefer, you can lightly salt the cucumbers and let them sit for about 10 minutes to release some moisture, then gently squeeze out the excess water.
In a small bowl, whisk together the rice vinegar, soy sauce, sugar, sesame oil, and red pepper flakes (if using) until the sugar dissolves.
Pour the dressing over the cucumbers and toss to coat. Let the salad sit for 5-10 minutes to allow the flavors to meld.
Garnish with sesame seeds and optional kombu seaweed before serving.
Notes
This salad can be stored in the refrigerator for up to 2 days. The flavors will intensify the longer it sits.