Acropora question (pic attached)

[kjdeut]

I have had this staghorn colony for over a year and it has been growing like crazy. It has always been a nice brown with the new growth coming in purple. As you can see in the pic one of the lower branches is turning a bright green, is this good or bad? If it was dieing I would think the brown color would just fade to white.
It has been under the same amount of light since I got it with the only difference being going from 10K to 20K bulbs. But, this is a bottom branch that doesnt get much light so Im not sure it the change in blubs would affect it or not. I have not seen any changes in any other branches.

Thanks for your help,
Ken

[Graham]

It's hard to tell, but the way the color change goes so abruptly, I think the green part is bare skeleton that has been covered in a green coralline-type algae.

When corals change colors, it is often more gradual and mostly throughout the coral, not just one branch.

[David Grigor]

To expand on Grahams: Do you actuall see polyps on the bottom areas ? Are you sure it isn't dead ?

Given that it isn't dead. It is not unusual for things to turn green under 20K ( Although you don't mention the wattage assuming this is MH, I only have experience with 400W 20K Radiums )
because of the spectrum or UV don't know which.........Although perplexing that it would be the underside and not the side face the light.