Polyp-feeding nudibranch

Dear Mark,
Your animal certainly looks like an aeolid nudibranch but it may be a species we don't know about. Most aeolids feed on cnidarians of some sort, some specialising on hydroids, some on sea anemones, or hard corals or sift corals. Most are very specific in their food choice. I am not sure what species your 'button polyp' is, but it looks quite like the one in a recent message, which was also being eaten by a nudibranch. Since we don't know what your slug is, I can't give you any advice on how 'dangerous' it is. However since you bought the live rock for the Caulerpa the button polyps and the nudibranch are a bonus. I am biased, but my advice would be to let 'nature' take its course, and just see what happens. It is possible the aeolid has quite a bit of growing to do. If so, we would have a much better idea of its identify if it were allowed to grow a bit bigger, and unless you find you have an invasion of these slugs, I doubt it will do much damage to the polyps.
If you are brave and leave it alive, I would be intersted in some followup information in a few weeks and some photos
Best wishes,
Bill Rudman