There are a zillion species of nudibranches out there. Without a pic nobody is gonna have a shot at identifying it.
Nudibranches are usually very picky eaters, so if you haven't added anything to your tank in the last month or so, it's obviously found a food supply in your system, as they won't live for more than a couple weeks without food so they are either a new addition or feeding.
If they have been in there a while, look for any corals you have that look like them. Predaceous nudibranches usually resemble their food for camoflage. If you have something that looks similar, check it over for nudibranches...