It's absolutely possible for an acro to grow over dead skeleton. I've seen new growth over dead skeleton with my slimer and red planet.
It looks like there may be live tissue there, just the polyps have been stripped away. Maybe predation? Is there anything that might be nipping at it?
As mentioned before, I would monitor for further damage, and frag out the damaged area if it seems to be getting worse. Otherwise, it looks like a very healthy coral.