IMO, I think it's just a ray of new growth.
In the past, I've had many millepora (appearing) corals just kind of sit idle, or grow very slowly. But, they suddenly get settled in and growth takes off. This idle period has lasted as long as a year with some corals. All of a sudden, conditions are right, and boom.
I have also seen the "Red Planet" do it too. The result was quite similar. Pale colored rays that gradually colored in at the base, while the leading edge stayed light in color.
Again, IMO, that's what I think is going on. But, be it mutation, alien, or devine intervention, growth is a good thing :thumbsup:
Well Done Steve!