Algae and Water plant

Algae noun - Primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves.

Water plant and algae are semantically related. In some cases you can use "Water plant" instead a noun "Algae".

Water plant noun - A plant that grows partly or wholly in water whether rooted in the mud, as a lotus, or floating without anchorage, as the water hyacinth.

Algae and water plant are semantically related. You can use "Algae" instead a noun phrase "Water plant".