Where'd that cloud come from...?

[Delphinus]

Sounds like your caulerpa sporalated aka "went sexual."

Unfortunately it just happens. When it happens to me, I just run some carbon, but basically, it just clears on its own in a day or two.

The one real kicker though, is that all the NO3 and PO4 that the caulerpa had sucked out of the water .... is right back in the water now. Some things don't do too well with a sudden jump like that, if the increase is big enough. So, watch out for stressed out critters, if it takes longer than a day or two to come back to normal then you might need to do a water change, or something.

That's the one drawback of caulerpa. Halimeda too -- although not as often. About the only things that don't sporalate, reputedly, are sea grasses/eel grasses, and that spaghetti type algae (super strong strands, tough like a brillo pad .... the proper name escapes me right now).