Thinly sliced avocado then toss slices in lemon juice. Top each cucumber slice with avocado and season with salt and pepper.
In a medium bowl, combine crabmeat, mayonnaise, Sriracha, and green onions. Season with salt and pepper, and squeeze more lemon juice into the mixture, if desired.
Top each cucumber sliced with a small scoop of the crab mixture. Sprinkle sesame seeds over each serving. Serve with soy sauce.