Rehydrate dried wakame in water, drain, and cut into bite-sized pieces. Remove the pit from umeboshi and mash into a paste. Julienne the cucumber. Prepare hotaru ika by removing eyes and beak.

Place frozen udon on a microwave-safe plate, cover loosely with plastic wrap, and heat according to package instructions. Rinse under cold water and drain.

Mix miso, mirin, vinegar, and mashed umeboshi in a bowl to create the sauce.