Place the wakame in a small heatproof bowl and add boiling water to cover it well. Set aside for around 5 minutes to rehydrate while you prepare everything else.
Trim the ends off the cucumber then thinly slice - using a mandolin or wide vegetable slicer/peeler will help get even, thin slices but you can also cut by hand. Place the slices in a bowl, sprinkle over the salt and toss gently to spread the salt more evenly (alternatively, place some of the slices in the bowl, sprinkle part of the salt, add more cucumber then more salt). Leave for around 3 minutes to let the salt draw out some of the liquid.
Meanwhile, place the vinegar and sugar in a small non-reactive microwavable bowl. Warm in the microwave for around 10-15 seconds so that the vinegar is warm and starting to steam gently. Stir so that the sugar dissolves.
Squeeze the wakame with your hand to drain off as much liquid as you can, without breaking the pieces too much then set aside. Drain the liquid from the cucumber - I generally press it against the side of the bowl with my hand then drain off the liquid, or else you can squeeze handfuls. Just take care not to break up the slices.
Place the drained cucumber, wakame and vinegar dressing in a bowl (either the bowl from the dressing, if large enough, or all in the bowl from the cucumber after draining works too). Mix together so everything is well mixed, but without breaking, and serve. If you like, add toasted sesame seeds on top as you serve.
Video
Notes
5oz/140g is approx 1 Japanese cucumber, 1 ½ Lebanese/Persian cucumbers or 2 mini cucumbers. If you use an English cucumber, you will probably want to remove some or all of the skin and some of the seeds.