It's been about 5 months now. Is it possible the coral is too close to the light? He's been in the tank probably a month and a half now and haven't had any issues till now.
Like I said, they can have small color changes.
If it has been in the tank now for a month and a half and is still showing polyps, I would think it is fine with the light.
As long as it doesn't start to rot and fall apart ( where the dark spot is), I wouldn't cut on it. The moment I saw rot, I would cut it off.
I would place it so it is standing up a little straighter,,,,,,,,,,,,,,,
Looks like a pinch point? Does it appear like its pulling from the top at that point? And I agree with mfinn, if you don't see it start to just dissolve away, I'd leave him be.