Just their presence on the porites means they are likely a predator of it. If you can see adults, you likely already have eggs.
What I would do: take the porites out of the tank and give them a dip that will kill the adults every 2-3 days for a few weeks. The idea is to kill them in the juvenile stage and not let them lay more eggs, then just trying to outlast the eggs that are already there. There are some better explanations about nudibranch treatment regimes out there for more specifics.
I don't know if dips, especially repeated dips, would harm the worms in the rock. There's certainly a chance of it, but unless you've got pretty active predators of the nudis, you will likely lose your porites to them (though, nothing else).
It's not going to be an overnight thing, and it's probably worth looking closely at the coral for tissue damage (and other porites if you've got it) as well as trying to get a more positive ID, but if it's a corallivore nudibranch, the known treatment regimes are usually pretty involved, and it's important to not just underestimate it since acting quickly will give you a substantial leg up. Just removing adults daily may afford you some time before you decide what to do, but I would expect to see more, and you will never remove all of them manually.
A writeup that illustrates this point:
http://www.seaslugforum.net/find/phesmino
That they have a veliger stage means that maybe heavy, fine mechanical filtration could help, but it also means that hatching eggs will spread throughout the tank and will probably only live to adults if they land near a food source, so killing the adults before they reproduce should be a reliable strategy.
