Depends. I believe Cheapjack was talking about movies and TV. I was mostly talking about book publishing. Not necessarily the same worlds or the same rules.
Where books are concerned, you can't copyright a mere.idea. "Vampire mermaids" is an idea, but nobody can clam dibs on that notion. Unlike, say, a specific novel,
Blood Splash by Joe Author, in which what is being copyrighted is the actual prose and execution, not just a high concept. You'd have to copy
Blood Splash almost word for word, chapter by chapter, before you'd be talking actionable plagiarism.
Otherwise, vampire mermaids has surely been done before and will be done again, without any copyrights being infringed. I throw the idea out to the world. Have at it!