Hm. Is that seaweed is edible by other organisms?
Then: more food -> more organisms -> more CO2
It's a level of middle school
Then: more food -> more organisms -> more CO2
It's a level of middle school
Which should be a hint to you that it's not anywhere near that simple.
Your body, like a furnace, would break the seaweed down and re-release the CO2.
Better sequestration, would be to grow the seaweed and then bury it. Then in a million years future Earth inhabitants could dig up that broken down seaweed and have a new oil rush.