Sweet,
I am not an expert on Nudibranches, but I belive that those like zoa eating nudibranches. Do they look like the have little "packets" or "pouches" on them, almost like little eggs, that are roughly the same color as the zoa colony they came from?
If so, they will eat your colony (which is why they were there in the first place).
I had a colony come with those on it, and I had to dip the colony to make sure they all died.
So unless you get a definitive opinion otherwise, I would not put them back in the tank, and I would remove any others as soon as you see them.
HTH
Stewart