if I see one bleaching, I usually try to cut off the bleached piece so it doesn't spread. I then throw this into the bottom of my tank where it can act as a substrate for other things to spread on, etc.
Sometimes, even months later, I see polyps reform on the bleached arm, usually starting up new coral growth from that point (ie., not spreading back to repopulate the original bleached arm.)
I've also seen some that bleached regrow and come back to life, but that was usually from 'coral fighting', where the bleaching and polyp recession is due to an unfriendly neighbor. I separate them, and it goes back to normal.
I guess a bleached piece can come back as started above by Faciosity, but I haven't had personal experience with this. I'm a noob though. Tank is less than a year old. When I've had a piece start bleaching due to lighting/water/disease, its usually been toast in my tank, unless some new growth starts off it way later. I only have one tank, so I've never experimented on saving the coral arm by moving it to a different environment.
