It's possible it got stung by another coral, I have such blemishes on various pieces due to me not securing everything in my tank with glue. Urchins, snails, hermits, and even some fish can knock things over sometimes and I'll find them later that day, one having clearly won and stung the crap out of the other.
Makes me wonder if anyone has ever done a "coral dominance" study about who the king of stinging is.