Alan, I think it came from the ocean....
Seriously, I had this stuff take over my frag tank last fall, and it smothered everything. I'm not sure how it got introduced to my tank, but likely from an addition of LR or some other new substrate (frag mount, etc). I don't think it's caulerpa, but I'm not an algae expert. My tangs used to eat it, but they've lost their appetite for it now.