looks like a pretty cool nudi if you ask me. how big is it? I have never heard of fish a eating nudibranch before. as mentioned the vast majority of them feed on hydroids, coral, sponge, tunicate or a mix of the above. the big grabby hand thing (i like your description
) would be its gill. my bet is that your dead fish were eaten by hermits and fish. that or just lost in your rockwork somewhere