In a medium bowl, mix the brown rice, mayonnaise, soy sauce, and rice vinegar together until all ingredients are evenly distributed.
In a deli container or straight-sided bowl, layer ingredients starting with the crab. Add rice, cucumbers, avocados, and remaining crab in alternating layers. Use the flat bottom of a similar-size container to flatten and compress each layer before starting on the next, so that it will keep its shape once unmolded.
Once your mold is filled to the top, carefully invert it onto a plate.
Garnish with cherry tomatoes around the bottom edge and microgreens on top.