Pretty sure Chinese Yam was Nagaimo (Huai Shan or Shan Yao in Chinese) while Yamaimo was literally mountain yam (or wild yam).
Though the texture is similar, apparently Yamaimo has darker skin than Nagaimo, though in some regions of Japan, people don’t differentiate the two.
I love it sliced into rounds and sautéed with a bit of soy sauce. Tororo on the other hand, depends on my appetite of the day.