i was talking to marti today about my hydraphora having a spot and she told me t post a pic so it may help. here it is the spot in question is in the center of the coral and it has a darker brown look to it, it is not on the surface. more like the coral has receeded from that area.
Looks to me like a dead spot that the coral has receeded and coraline algae has started to grow on. Watch and see if it gets bigger, if so you may need to frag the good coral away and start over.