Green algae and Pond scum

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

Pond scum and green algae are semantically related. In some cases you can replace phrase "Pond scum" with "Green algae", this phrases are similar.

Pond scum noun - Free-floating freshwater green algae.

Green algae and pond scum are semantically related. You can use "Green algae" instead a noun phrase "Pond scum".