A new, too-intense light could easily bleach both chaeto and caulerpa, but caulerpa may just need to be pruned- if you have chaeto, raise the light, make sure the ball of chaeto has flow around it and turn it over now and then if it's not tumbling around on it's own in there.
You may possibly even have a real lack of nutrients (a good thing) which would cause somewhat of a die out; your nitrates would be zero for quite sometime and you'd notice no other algae was making it, either.