Touch it and see if it's hard. It almost looks like a baby Fungia, which grow on small stalks on rocks before breaking off into plates. If it's squishy and definitely doesn't have a skeleton it could be a Majano anemone, which you should kill with extreme prejudice.
I'm confused on how to tell the difference. If its a plate it would be hard at the base? The good news is the rock it's on is loose, so I could take the whole rock out & do what needs to be done without messing with the DT
It's a little blurry. The Zoas don't look like they're being stung. Keep an eye on it closely and don't disturb it. If it's neighbors start getting stung, it's gotta go.
Don't poke at it too much. It'll multiply and if it's aggressive you've got a real issue on your hands.