bergeii ? You mean Berghia.
Aeolid nudibranch, preys on cnidarians - Corals and anemones.
Might prey on other things such as bryzoans, tunicates, and sponge.
It is no good if a reef, probly mildly toxic, nothing your system should notice. They are very easy to keep, If you decide to keep it, offer it a wide array of softie corals to feed on.