Unfortunately, that acro looks deceased. The brown flaking stuff looks to be its skin, which has all peeled off. There are many reasons acros (or any coral) will die like this. Rapid tissue necrosis (RTN) is not uncommon, especially for acros since they are tougher to keep. It could be parameters, too much flow/light too soon after acclimation, insufficient DKH/calcium parameters, too high phosphates/nitrates, etc.