Japanese mustard is not the wasabi, but karashi, a condiment often used in Japanese cuisine. Presentation:
Karashi, the must-have sauce for natto
Karashi からし is made from brown mustard seed. It is yellow or ochre in color, and its taste is a bit like the European mustard, but more pungent and with a “root” aftertaste. It is sold either in powder or in tubes. It owes its success to natto: It is made of fermented soybeans which give off a very specific, even unpleasant smell. The Japanese eat it for breakfast. The natto is energetically mixed with karashi sauce with Japanese chopsticks.
In Japanese cuisine
In Japanese cuisine, it is used as a condiment:
Tonkatsu 
A Japanese dish made with fried breaded pork. Karashi is placed on top with Bulldog sauce.
Oden 
It can be compared with the French pot-au-feu, you will find fritters, fish, vegetables… Karashi goes well with oden.
Nabe or Japanese fondue 
Karashi sauce accompanies nabe, a Japanese fondue in which meat, vegetables, fish, tofu…
Natto 
These are fermented soy beans, karashi is used here as a condiment.
Lotus Karashi: karashi rencon からし蓮根 
A specialty of Kumamoto city, it is a lotus root with a paste made of miso. It is a lotus root with a paste made of karashi mustard and honey. The whole thing sits for over 5 hours, is fried, and then sliced.
We use S&B brand karashi in a tube that you can find in Asian grocery stores, it is very good!
I just had karashi honey mustard from a katsu place and the spicy shot through my sinus like wasabi.
Hello Anei, there are several types of karashi, some milder than others, but like mustard and wasabi, you should avoid using too much.