The main RuneScape Wiki has an article on: Edible seaweed
The Old School RuneScape Wiki also has an article on Edible seaweed.

Are you looking for regular seaweed?

Edible seaweed is a type of food that heals 4 hits. It is uncommonly used or traded since it may only be obtained after a successful catch for the Fishing Trawler minigame. Acquiring the seaweed provides the player with 5 fishing experience.

As the name suggests, it can only be eaten and does not serve in the making of soda ash.

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