The Spawn of the Green Abyss are a fictional race expanded upon in 1994 by Chaosium for the Call of Cthulhu roleplaying game, which was based on creatures from C. Hall Thompson's short story "The Spawn of the Green Abyss".
The Spawn of the Green Abyss are a highly evolved and intelligent race which lives in the oceans and seas of Earth. Living in societies ruled by their queen, Zoth Syra. They appear as bluish slime whose various features are shifting. Limbs and sensory organs are created as needed. The songs of the spawn can influence those whom hear it. Though normal weapons cannot hurt them, they can be harmed by fire, chemicals, electricity, magical spells, and enchanted weapons.
Behind the Mythos
In the actual short story, the creatures are not described at all in the same terms as the Chaosium version. In the short story the creatures are clearly not human, but are at least semi-humanoid.