The second seaweed we get is 紫菜 (Mand: zǐ cài; Cant: zi2 coi3), literally 'purple vegetable'.
This is Porphyra umbilicalis or laver. It turns up in all sorts of places, though we don't get it fresh. It comes in circular packs of dried weed like this:
This is used in soup, often with seafood or with egg.
Laver and egg soup
These soups are sold as instant soup mix everywhere.
Processed using a technique similar to paper-making this becomes known as 海苔 (Mand: hǎi tái; Cant: hoi2 toi4), which sushi fans know of as nori, used for wrapping sushi rolls etc. It is used the same way here - yes, China loves sushi. After all, they invented it!
Nori style seaweed is also seasoned in various ways, often with soy sauce and sold as a snack item. Every supermarket and corner shop has it.
It also features in many other snack items.
and even Lay's make seaweed chips using it.
as does the maker of my favourite crackers to use with cheese.