I don't know what this is.

[Leverage Lad]

Any body know what this is? It came in on some Zoanthids and is stuck to the glass. I have counted at least 20 arms(?). He looks like he is just stuck to the glass cleaning it. Window cleaner might be a good name.

[dobejazz]

No idea but cool looking in a creepy sort of way

[hng]

looks like an aiptasia anemone to me.

[steamboatwv]

aptasia is my opinion also, just got rid of my infestation of them

[Loverotties]

I'd get rid of it also!

[ChilisREasy]

its aipstasia (acrozoanthus) considered a pest cause there brown & propigate rapidly, many ways to kill them safely..never crush them as that just spreads them..butterfly fish, peppermint shrimps will feed upon them. if that fails..youll have to use chemicals.