Algae and Green algae

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

Green algae and algae are semantically related. In some cases you can replace phrase "Green algae" with "Algae", this terms are similar.

Green algae noun - Algae that are clear green in color; often growing on wet ricks or damp wood or the surface of stagnant water.

Algae and green algae are semantically related. You can use "Algae" instead a noun phrase "Green algae".