Tales and sightings of Merfolk have been recorded since the beginning of history. It's said that few humans can resist the spell of the Merfolk, which is powerful and overwhelming.
Many a sailor has been drawn into the sea by the gaze of a mermaid, and Triton, the male of the species, can shed their true form and walk for a limited time among humans.
Though Merfolk in general act kindly towards humans, their motives are mysterious. Those who have dealins with them do so at their own peril.
In the episode Fathom Jack is very suspicious of his mother's new love interest and well he should be since the man is really a merman.