Seaweed is a very confusing term, yes. Should be used for marine weeds [upper plants with flowers], but the term refer to algae that are a different category.

Seaweed is a very confusing term, yes. Should be used for marine weeds [upper plants with flowers], but the term refer to algae that are a different category. Some algae are plants. Other are neither plants, animals or fungi and have their own category.