Combine avocado, yogurt, mayonnaise, wasabi, lime juice, and salt in a food processor; process until smooth. Place mixture in a large ziplock bag; snip off 1 corner of bag to make about a 1/4-inch hole.
Slice cucumber diagonally into 32 1/8-inch-thick slices. Pipe a pea-size amount of avocado cream onto each cucumber slice; place salmon on top of avocado cream. Pipe remaining avocado cream (about 1/2 teaspoon per slice) on top of salmon. Top with microgreens and, if desired, caviar.