Good question! Genocide is not just another word for mass-murder. It is defined as killing with the intention to destroy the targeted ethnic group/culture/religion completely. That certainly wasn't the American plan anywhere during WW2. Germany's actions were obviously genocidal, and Japan's actions in China arguably were as well.
Haha yeah, I was reluctant just because I don't personally know much of that particular history and it was hard to imagine that the Japanese intended to commit genocide against all of China, but I agree that genocide is fair classification of their actions.