It is going sexual. If the amount of white area is very limited, I would not worry about it. Mine in my fuge does the same thing periodically. If the amount of caulerpa in your tank and fuge is small, there is nothing to worry about.
However, if you have a lot of caulerpa and see tiny stringy things coming out of a large part of caulerpa or the whole caulerpa turning whitish, WATCH OUT.
Here's a short and easy to read article about caulerpa going sexual. You can see the little stringy things that I am talking about in the pictures here:
http://reefkeeping.com/issues/2004-05/nftt/index.php
If you, our new members/beginners, are wondering about this, you can read on. Otherwise, just disregard the part hereafter.
When caulerpa goes sexual, you may end up with your tank looking really green with their spores. If this happens, I recommend that you do a sizable water change or at least aerate your tank right away.
The entire batch of caulerpa can go sexual all at once. The resulting tiny green spores respire and exhaust oxygen in the tank - this is the fish killing event that people often talk about. From what I understand, dying caulerpa does not release a high enough amount of caulerpacin or turpentine like toxins to kill the entire tank. It's the hypoxia resulting from the spores taking up oxygen for photosynthesis that kills other tank inhabitants.
Although there are a lot of theories about why Caulerpa goes sexual, according to my friend, Tom Barr, who is a marine biologist that specializes in both fresh water and salt water plants and algae, the trigger mechanism seems to be largely related to the fluctuation of the nutrients level in the aquarium water. Massive die off is its ditch effort for survival - releasing spores to preserve the species. Other bad stresses, such as huge swings in pH, salinity, and temp, may trigger the same response, though.
Tomoko
