Probably it was caulerpa, which does do that on occasion.
I wouldn't add caulerpa directly to your display; it tends to go weedy, and may go 'sexual' (ie, turn white and decompose). It's best kept in a sump lit 24/7 so that it can be controlled. If you have the time to keep it pruned, though, it can be an interesting addition to a display. Comes in lots of different shapes.
Chaetomorpha or gracilaria probably would have been better choices for nutrient control...make sure light and flow are adequate for macro growth as well.
