I can help you a little. The snail is a buccinid whelk, probably either a Gemophos or Cantharus or Pollia. They all behave about the same, and well predate on something, usually barnacles or other snails. The thing that looks like a nudibranch is..a nudibranch. It appears to be one of the sponge-eating dorids, so it shouldn't be a problem in your tank.
Cheers,
Don