Several things can cause spontaneous expelling of
Zooxanthellae or
bleaching. Stress in the process of going to a
frag swap, ie, bagging, sitting in bag for a long period, getting jostled around, transfer to new environment, too little or too much light in new environment. If the
bleached area still has polyps there's a good chance it will recover. Watch it carefully over the next few days. Be prepared to move it up or down in the water column to get more or less light.