Chop the shrimp in a food processor. Add the remaining mousse ingredients and process until the shrimp are finely chopped. Remove the shrimp mousse to a bowl and mix rapidly to incorporate air into the mixture.
Cut the block of tofu in half horizontally; cut each piece in half lengthwise, then crosswise, to make 8 equal pieces. Lay the tofu pieces side by side in a 9-inch glass pie pan or other heatproof dish. Evenly spread shrimp mousse over each piece.
Place a steaming rack in a wok, add water to just below the level of the rack, and bring to a boil. Place the dish on the rack; cover and steam until the shrimp mousse turns pink, about 6 minutes. Remove the dish from the wok; carefully pour off the cooking juices. Drizzle soy sauce and sesame oil over the tofu; sprinkle with green onion and serve.