What could this thing be?

[mikelush78]

I started cycling my 90 gallon aquarium and on the Live Rock that I got this thing was on the rock and I am wondering what it is...
Its about the size of 1/4 of a penny, and the middle is red and the outside is bright green. It moved in 24 hours about 3" on the rock.

Any guesses...

 

[Fishieness]

looks like a jewel anemone. they are very common and will move to a darker spot under the rock.