Perhaps a gorgonian feeder?

Dear Jon,
It is certainly an aeolid nudibranch but not one I recognise. The only gorgonian feeding aeolids belong to the genus Phyllodesmium. Many species of the genus are solar-powered through their removal of zooxanthellae from the soft-corals they feed on. Your animal has a brown network on the foot and body, which suggests that it may have modified digestive gland ducts to harbour zooxanthellae. However species of Phyllodesmium lack cnidosacs, but in your photos there seem to be obvious cnidosacs at the tip of the cerata, so it is unlikely your animal is a Phyllodesmium. 

One other possibility is that your aeolid belongs to the family Aeolidiidae, like the species of Aeolidiopsis which feed on Palythoa. In the lower right photo I have ringed one of the rhinophores, which seem to have little tubercles or branches. Species of Aeolidiopsis have branched rhinophores.

I may be wrong but I suspect your animal is not eating your gorgonian but it may be eating Palythoa, if you have any in your aquarium. I know you will probably just want to squash it, but unless it is part of a large population in your aquarium, it shouldn't do much harm on its own. I would very much like to know if it grows much bigger, and what it actually eats, before you kill it.

Best wishes,
Bill Rudman