An Algea was a creature that fed on grief. It spoke ancient Greek. The Algea was related to Oizys, the Greek god of anxiety grief and depression.
One Algea, who looked like a raven-haired woman, was locked away by P.R.o.B.e. agents Giles and Archie. She was murdered by Tasha with a branch from an olive tree dipped in lamb's blood. (PROSE: The Door We Forgot)