Horror, female, mermaid, other(I'm imagining a flute, or another delicate instrument that would work underwater).
The deep dark ocean is already a scary place, so populate it with a few leviathans, giant squids and eldritch monsters, their form hidden by the darkness of the abyss, and you can easily evoke existential horror.
In this world, mermaids are a migratory species/culture, and that the MC has gotten lost and separated from the group by the attack of a sea monster.
She is left alone with only the ability to talk to sea animals and a magical instrument that can charm low level sea creatures. Relying on vague instinct, she sets out to find her tribe again, traversing the abyss blind, without the wisdom of her ancestors to guide her and help her avoid the dangerous places.
This is the story of a sheltered and pampered girl that is suddenly thrust into an unforgiving world, to make terrifying enemies and lifelong friendships. Even if she survives, will she manage to keep her innocence?
The mermaid and her animal friends are very cute and full of personality, but there are no villains or bad guys. She is facing huge, unthinking monsters, deadly currents and unforgiving storms, all mindless dangers that make it seem like the entire world is conspiring to kill her.
For her, it's not only about surviving, it's about remaining sane when confronted with loneliness, loss, and despair, living in a constant state of exhaustion and fear, with no end to her journey in sight.
The idea is to greatly contrast the MC and her friends as pure, lively and cute, with the inexorable forces of the universe that are unfeeling, relentless and unstoppable. The world may not be aiming at her specifically, but one writing step is all it would take for it to be all over.
