Looks a lot like the nudibranch that showed in my tank. And yes it does eat soft corals. The one I had was mowing down my colt corals until I could catch him. The one in my tank was about 2 inches long pure white, had frills on his kind of like a lettuce nudi, and had a pointy tail. I ID'd mine from the Marine Invertebrates book I have by Ronald Shimek. The nudi is on page 348.