This happened to me as well. It never happens to cheato and that is why some people never keep caulerpa
I found this :
Caulerpa spread almost entirely by asexual growth from spores and by fragmentation. A spore producing Caulerpa can often times release such large numbers of spores that it turns the aquarium water a milkly-green color. If this occurs the spores will settle out, be removed by filtration, be eaten or die off in a short period of time. The water becomes clear again, but the spore producing alga can leave behind a white soft tissued husk that should be removed.
At this site:
http://saltaquarium.about.com/cs/alg...a/aa112000.htm