Our sincerest apologies, Lee -
All of us here at Manhattan Aquariums strive for rigorous quarantine process when it comes to livestock and I'm very sorry to see that such a hitchhiker came with your purchase. When dealing with such heavy volume of water, rocks and corals from all over the world on a consistent basis, it will always be an on-going effort for us to make sure we are as close to pest-free as possible when the corals finally reach our customers but unfortunately, few will slip through our radar occasionally.
We do ask of our customers to take the precautionary step as well when dealing with any livestock is added to their system by going through the process of dipping and QTing.
When dealing with nudibranchs, 20 ~ 30 mins dip in Melafix or FWE will kill them. Coral Revive is another excellent product to use for preventive dips. Afterward, a quick dip of a minute or two in fresh RO, shaking the frags / colony rigorously will ensure that any remaining nudibranchs will come off. Repeat this process every week or so to account for any possible eggs remaining on your coral and it's a bullet proof method of getting rid of these guys. Especially in smaller systems, it will be a very easy process for contain and control. Hope that helps,