it's just common growth. when an sps coral grows too fast the tissue can't keep up at the same pase and thins and gets a white color, i've had this happen to a lot of my corals too.
Had a bad infestation of algae a few months ago! I have now got it under control...well that's an understatement. It's completely gone and I now have some nice coraline on the rocks
Here is a pic of it in July when I got it (you can see that little nub)
Here are some not so good pics I took tonight
And since we're talking about growth....here is my little pearlberry frag I got at the begining of Aug (if I remember correctly...or atleast that's when this pic was taken)