Hard to be 100% sure from the pic, but it LOOKS like a majano anemonia...
They look kinda cute as one or two, but can quickly and easily spread to plague proportions... If it is indeed a majano, I'd terminate with extreme prejudice...
Here's a Google Images search on "majano"... Look at these pics and verify that's what you've got before dispensing death from above though!
http://tinyurl.com/5sa24v