What's the name of that tool used to make mochi?

It is a stone mortar.

The stone mortar, a tool for pounding rice, is four-sided, wide at the top and narrow at the bottom, resting in the earth, with only the mouth of the mortar exposed to the ground. With the wooden pestle and mortar up and down the bump, rice, but also in the stone mortar shell, molted skin, revealing the dark red snow-white grains of rice. Smaller stone mortars can be used for smashing, pounding, researching and grinding medicinal herbs.

It has been proven that about three thousand years ago, the Chinese began to eat rice. From that time onwards, stone and wooden pounding began, until the invention of the rice mill in the middle of the 20th century, the stone mortar gradually withdrew from the "stage of history".

Expanded Information:

Mochi is made from glutinous rice and potatoes, soaked in water and steamed in a steamer basket, then quickly pounded in stone scoops until soft and pliable, and then made into small or large clusters while still hot, and then rolled in a pan of sesame seeds fried in flour and mixed with sugar (or soybeans fried and ground in flour and mixed with sugar) to be eaten.

The taste is fragrant. Now the street vendors more pedaled tricycles selling, rice mud with good insulation performance of the special iron bucket put, shake the handle, rice mud from the round hole drilling. But where there are happy events, the locals have to do brown sugar patties to entertain guests, in order to show good luck.