Same thing is happening to me, you have to have enough DPS to kill him. After fighting ghost for 2 hours I was finally able to time my full burn DPS timers to get him down before he healed back up to 24% again.
I'm actually having this Regen/healing problem in all zones and not just the new event zone. Raid bosses always heal back to 24% if I don't kill them fast enough. This is very frustrating.