I've heard that they will only eat the tiny aptasia's....which is just the experience we've had with them. We put a bunch in our 120, but unfortunately, I think our Sally Lightfoot crab might have gotten one or two of them.
I have, however, just been doing some reading on a species of Nudibranch....Berghia....and supposedly all these guys eat is aptasia. I just put an order in for a bunch of them.....we'll see.
M