Halcyon by Demelza Carlton

Catch a siren. Stop the storm. Whatever the cost.
Allie, sick of war and grief, signed up as a lowly maintenance worker for the Colony in the hope of securing a lasting peace between the Titans and Humans. However, the girl has her own secrets, not least of which is her assignment from the New Hope government to monitor the Colony for signs of a suspected saboteur.
Galen, the only survivor of the Poseidon shipwreck, didn’t join the Colony for peace. He wants vengeance on the siren Halcyon, the mysterious Mer whose terrorist tactics single-handedly killed more Humans than any other Titan. His sources say that Halcyon is on New Hope, and he’ll happily kill every Mer in the Colony until his parents’ murderer is dead.
Can they catch the terrorist before the Colony is destroyed?