Noodles with pickled mustard and pork @ Din Tai Fung

The green vegetable is mustard?

It's pickled mustard greens. It's a green veggie that the Chinese like to pickle and add to dishes. It's got quite light umami and is a bit sweet.

I googled mustard green but it was leafy vegetable. Pickled mustard in your post looks more like daikon ( Japanese white radish ). Are there any typical dishes cooked with this?

Yes it's a leafy green vegetable. The mature vegetable has big stems which is why it's a favourite to pickle. It gets crunchy. Chinese use it in stirfries and soups.