Is a seaweeds is a plant?

No, it isn't! Seaweed is a type of algae. Algae lack the traits used to classify plants as terrestrial: cuticle, stomata, roots, vascular tissue, or leaves Plants use cellulose with lignin in cell walls (Algae lack a cell wall or have different components like agar or alginic acids mixed with cellulose.) Algae also have little sexual reproduction and do not have a multicellular embryo like the plants have.

All algae have sperm as the motile form in their life cycle. The majority of plants have lost this trait and use pollen not sperm.