After reading all of this, I am convinced that I have one aiptasia. I had quite a few on rocks when I first started my tank four years ago (came in on live rock from a "pet" store). Over time, they all disappeared - except - I now have one huge one - it is about five inches long when extended. It has attached itself to the glass near the light - it's "foot" is over an inch in diameter. It occasionally moves, but mostly just stays there. I have never had anything disappear (shrimp, fish, snails, crabs, etc.) that I attribute to the aiptasia. I will even admit that I started feeding it about a year ago (krill). I have never since seen any others (2+ years). I have read all of this horrible stuff, and based on the pictures, I really do think it is aiptasia. I guess I know what you all are going to tell me - kill it, now, before it spreads - but... Well, some of you probably understand.
Also, I have a ton of bristle worms in my sand and rocks. I have read the threads that say they're good cleaners and mostly just shift the substrate. So, if they're not bad for the tank - how come everyone (including Saltwaterfish.com) sell the bristle worm traps???
Also, I just added some snails to my tank and I found bristle worms eating one on the bottom of the tank. I assume it was dead and they found it. Will bristle worms attack the slow moving snails or hermit crabs I have added to my tank?