Tritoniopsis elegans - Soft coral eating nudibranch

I had this nudibranch living in my aquarium for 3mthsI The colt coral it came on started off approx 6-7" tall, and i positioned it close to a nearby rock; hoping it would move onto it. Instead I thought id damaged one of its arms [it appeared to go white and look dead-like, which in actual fact I now realise was this thing!

If you look closely you can see the camouflage that mimics tiny colt coral branches.

Now the colt is recovering, but is only about 4-5" tall. At the time i was about to assume I was not giving it enough light or food... in fact it has been eaten alive -