It is a flatworm not a Nudibranch and is most likely alone.
They are actually more common than you'd think. Most often feeding on detritus, Ascidians, clams and other items in your reef. Often they are harmless but little is known about their dietary habits and they may snack on alternates in the aquarium when their regular food is not available.

Nudibranchs are more of a concern as they are often much more destructive in their feeding habits.