I can't believe I am even questioning Boomer
and while it could be the nudibranch ID, I still say it could also be a flatworm. It looks like there is another flatworm at the top right corner of the picture as well. When you say, "they are like rabbits" are you reffering to their reproduction rates? If so, then I would almost assuredly say you have the red flatworm.
Its tough to tell by that picture but just a hunch on my part...
Take er easy
Scott T.