Is this one still housed near the other, even though they're in separate cages now? Igs can be very territorial, and if there was stress between the two, it will continue as long as they can see or even smell one another. Is that how this one lost the section of its tail? Fairly common thing to happen. And yes, iguana tails do regenerate, they start out black and will eventually turn green again, though without any pattern, and they will look slightly different than the original. And it's a slow process.