Even under normal conditions, there is always a regular expulsion or digestion of "old" or dysfuctional zooxanthellae. Such regeneration is not bleaching, and the observation of strands of brown zooxanthellae being emitted from the a coral are not neccessarily indicative of a bleaching event. (E. Borneman)

I'd say you had a small bleaching event, but it could also just be time for your zoo's to renew his supply of zooxanthellae. One way or the other a water change and carbon couldnt hurt!!!