Letter of recommendation from professor who is not longer at your university
It's not just a good idea, it's an excellent idea.  Your professor is still in just as good a position to speak to his experience with you and his assessment your abilities as he was before.  You say he has moved to another good institution, so there is no reason for his opinion to be discounted just because he moved---changing institutions is not unusual.  In fact, if he is happy to write you a good letter, it will look even better than if he was at the same institution, because even after changing institutions he still remembers you and cares enough to write.