sea weed is a very generic term, it could mean many different types of sea plants.
From wikipedia "Seaweed is a loose colloquial term encompassing macroscopic, multicellular, benthicmarinealgae.[1] The term includes some members of the red, brown and green algae. Seaweeds can also be classified by use (as food, medicine, fertilizer, industrial, etc.)."
So yes if you want to classify cheato or whatever is floating in your tank as a weed, then yes it's sea weed.