Some red, green and brown algae that grow inside other plants are termed endophytic, and some of these are also parasitic. Others grow inside animals and are called endozoic.
One group of primitive organisms, Cyanobacteria, can also grow endophytically or endozoically. Microscopic examination is required for precise species identification, but specimens may be encountered by the casual naturalist.

No relation
Miscellaneous Marine Algae