Apparently that’s not obvious when you have not seen it done. So a few photos are better than a long speech.
This veggie nagaimo is slimy and if you peel it it becomes very slippery, and you may get some skin allergy into the bargain. But that’s only after it’s peeled.
You can touch the skin without problems. So with a knife, peel a length of that long potato, or the tip if it’s the round type. Like a pencil, you sharpen it as you need.
Grating the in the traditional way in a suribashi (mortar). and you rub it against the lines of the pottery.
Second way, with modern graters.