Caulerpa -- where does it come from?

[Matt]

Tank is about 18 months old, and it has been more than 4 months since anything except water changes and food have been added to it.

Suddenly, I'm getting growth of feather caulerpa. Seriously, does this stuff come in with the photons?

ARGH. When the tank was new, I had a small infestation of grape caulerpa, and it took ages to beat it down.