Frisbee is right on the money.
It is an Aeolid nudibranch. Almost all of the nudis in this family feed on anemones (some on hydroids). This family includes the aiptasia eating nudi.
An interesting thing about nudis is that they can look completely white/opaque and then after they eat can color up beautifully. Yours has eaten recently, it has great color on it. The reason why they color-up is b/c they take in and incorporate what they eat into their system (zooxanthellae, nematocysts, etc.). On the one you have you can see is the zooxanthellae in its tentacles (Cerata/ceras). And the yellow tips, these contain some of the stinging cells (nematocysts) from the cnidarian/anemone. They then are able to reuse them in their own defense.
Did you get your rocks from the Gulf or Caribbean area? If so, then I believe what you have is this guy:
http://www.seaslugforum.net/find/bergverr
There seems to be a name issues going on with the Aip eating nudi which is this one:
http://www.seaslugforum.net/find/aeolstep
Both are from the Caribbean and it sounds like both may eat Aips based off this discussion:
http://www.seaslugforum.net/bergverraq.htm
[/url] image by jessica_haering, on Flickr[/IMG]
