Lychee-Lime Dressing: Combine all dressing ingredients in a blender or food processor and pulse until liquefied. Transfer to a clean glass jar and refrigerate. To Assemble: Wrap the purple yams in foil and bake in a 375°F/190°C oven until you can pierce them easily with a fork, about 45 minutes to one hour. Allow to chill thoroughly in the refrigerator then peel and cut them into ¼-inch/0.5-cm cubes. Ten minutes before serving, in a mixing bowl, sprinkle sliced cucumber and cubed jicama with a pinch of salt and let sit for about 5 minutes. Gently add corn and yam and toss together with a splash of oil. Divide salad onto small plates. In a glass mixing bowl, add about ¼ cup/60 mL of the lychee-lime dressing to the sliced geoduck and mix well. Allow to stand for 5 minutes. Top each plated salad with a portion of ceviche and dressing. Spoon a bit more dressing onto each plate and serve.
