Leave it be. Some acropora will grow back over their base faster than others. One thing it will NOT do, is repopulate that skeleton. It'll just grow on top of it. I've got some montipora that have had spots of STN in the middle of the colony, that died off, then stopped, but while the coral continues to grow laterally and encrust around it's perimeter, it NEVER regrew over that original dead tissue. Rather unsightly