When I was dipping a zoa rock this morning to be safe, I noticed these under the rock. ????
The stuff under the rock just looks like pineapple sponges. If the nudi is a coral eating variety, you'd probably see them near their food of choice, and their eggs would probably be very close. Not to say they can't roam, but they often stick near their food source. They often take on similar colors to their food, so if you have a coral that glows like that, maybe that's your first place to look for them.
That said, if it was that big and you haven't seen any coral losses, it may not be a zoa eating kind, there are many species and typically different species have pretty specific diets, so it could be after something else (and non coral is a possibility too).
If it is a zoa eating variety, you could try pulling any zoas from your tank, dipping them, and keeping them in QT. If you then dip them every few days, you should be able to kill the new nudis before they get old enough to lay eggs, so after several cycles you should get rid of them. Since zoa eating nudis really only eat zoas, if you pulled them all out of the main tank, then any remaining in there would starve, and after a period of QT dips and a zoa-fallow display (probably several weeks), you should be rid of them.