Algae and Red algae

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

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

Red algae noun - Marine algae in which the chlorophyll is masked by a red or purplish pigment; source of agar and carrageenan.

Algae and red algae are semantically related. You can use "Algae" instead a noun phrase "Red algae".