Actually seaweed is a term given too many different types of marine plants that grow in the ocean and none of them are weeds, in the sense that we would try to get rid up them with a weed killer.
The basic scientific term would really be algae. Algae (Red, brown, or green) are a very large single celled phytoplankton.
Phytoplankton in the world’s oceans is extremely important due to their immense numbers.
Did you know there is more phytoplankton producing oxygen and absorbing the carbon dioxide than there are trees on the land?
Do you have another great question? Check out www.beachchairscientist.com and enter your request!