Need an ID please
 

Hi everyone. I don't post much but I do lurk the boards and thought this would be the best place to get some help IDing something I found in my tank today.
My tank has been up and running for over a year and I recently discovered that I had a Mantis shrimp living among my live rock. Today is the day that I decided to pull out the rock he is living in and get him out and find him a home. While doing so I came across a rock that had a very odd looking 'thing' on it. After much hastle I finally removed the creature from the rock he was living in and I have no idea what it is! It almost looks like some sort of budibranch but I can't find anything that remotely resembles this thing. It is tan and cream in color and moves much the same way as an octopus does. I don't want to put it back in my tank if it is harmfull and not knowing what it is has me kind of worried. I have taken some pictures of it and they can be found here:
http://photobucket.com/albums/v310/T...t=DSCF0016.jpg
The picture isn't the best and I will try to get some better ones. This thing is moving about the dish it is in like crazy!
Any help would be greatly appreciated. Thanks
ETA: Mantis Shrimp is still in the rock and refuses to come out. 1-mantis 0-Me

Anything on this page look familiar?
http://www.wetwebmedia.com/flatworms.htm

Was hard to tell from your photo, but one looked close to me.

As far as the body shape, they look very close. Just a different color pattern.
I am assuming that if this is a type of flat worm, then I probably don't want to put it back in my tank? It is very cool to watch it in the cup it is in but I have never seen it in the tank until now and doubt I would see it again if it was something I could/should put back into the tank. Thank you for the link though. At least it narrows down what it could be :)

I've had these before in my tanks. I stumble across them when moving rocks or looking at the undersides of rocks and coral.
It seems to be a Pericelis sp.
It seems to be a predator. Most likely feeding on micro fauna and small copepods. It does not appear to bother corals.