Title says it all.
I have found a beautiful sim that I would love to turn human I can't find a way in cas and I found a cheat as well "traits.remove_trait trait_OccultMermaid" so I used the cheat however it doesn't work properly, her mermaid part disappeared..great..however instead of the water changing back to hygiene it still stayed as hydration and mentions the mermaid needing water etc..Is there a simpler way to just press "x" in cas? Am I missing something? Thanks in advance.

Not sure where you find mermaid kelp, but I found this on the internet: "To turn back to a normal Sim, all you need to do is eat the Mermadic Kelp once again, but this time you need to eat two. This will remove any of the traits and abilities they had with their transformation, so keep this in mind."

Mermaid kelp can be bought from the rewards store in-game for I believe 500 satisfaction points (I will need to check to verify the amount), but I also believe I saw elsewhere (either Twitter or the official forums) that exploring the cave in Sulani has a slight chance of giving mermaid kelp.

I found a gorgeous Sim too (Crystal Speldulva or something like that). I ate two kelp, and in CAS it doesn't show she's a mermaid...but she still has hydration instead of hygiene. Ugh.

Try using cheats to remove her hidden traits
open the cheats console and paste those (hit enter after each one)

traits.remove_trait OccultMermaid_Discovered
traits.remove_trait OccultMermaid_MerfolkWoke
traits.remove_trait OccultMermaid_MermaidForm
traits.remove_trait OccultMermaid_TemporaryDiscovered
traits.remove_trait OccultMermaid_TYAE