Zoa eating Nudibranchs usually come in on colonies or plugs of Zoas. They take on the color of the Zoas that they are eating, so they are difficult to spot. If you notice that your Zoas aren't opening, or some are starting to be missing, you may have Zoas eating Nudibranch. Zoa eating Nudibranchs lay their eggs on the Zoas storks, and are white and in the shape of a spiral.
Fresh water dip will kill zoanthid eating nudibranchs, though they wouldn't have any affect on the egg, which need to be remove or destroyed separately. Simply dip the affected zoas in fresh water and blast them with a turkey baster, this should blow the Nudibranch off of the Zoas. Zoa eating Nudibranchs can also be removed by hand with a pair of tweezers or using a turkey baster to suck them up. It's important to do fresh water dips to all your Zoas, and also check for eggs, before you place them into your main tank.