You Sim is stuck mid-transformation so he/she is not either human or mermaid, you need to get in the water and then cheat he/she into a mermaid then remove the occult and he/she should be normal again, it took me all day to work out so I hope this helps you all ☺️ (Cheats are underneath)
You can add the Mermaid Trait to your Sim using the above testing cheats then typing Traits.Equip_Trait Trait_OccultMermaid. Similarly, you can return a Sim back to human by using the Traits.Remove_Trait Trait_OccultMermaid cheat.
