I havent experienced any underside bleaching yet. That would make me nervous too, but yours looks like a minor problem considering the other corals in your tank. But im sure you have seen tanks with thick branching and absolutely no bleaching anywhere.
Although many reefers use the terms loosely me included, there is a distinction between stn and bleaching; and should be approached differently. Bleaching can lead to "stn" (slow tissue necrosis), which is the actual dying of tissue but that can also be a symptom of many different things.