I'm no nudibranch expert, but don't they contain toxins for self-defense? I know some nudis do, but I'm not sure if they *all* do.
If so - and I realize this is a little late since your Elegance has obviously finished the guy off by now - I'd be worried about the health of the Elegance, or what happens when the Elegance spits the remains back out.
Granted, nothing much to do now but sit back and wait... but I'd have a fair amount of water ready to go for a water change if you don't keep it onhand as a normal practice.