It actually looks like a australomussa to me, same overall family. I have healed scolly before, cleaned area with dip. But what caused the damage. If from another coral it cam become worse. It sometimes takes a day or so for damaged flesh to show up.
Which one moved? Guessing the scoly since they get more puffy than australomussa. I have a cynaria which puffs himself up to move like a plate coral will. It must be a family group thing. The dip will help clean any possible infections. Feedings will also quicken the healing time.