Halimeda does absorb calcium from the water much like stony corals do. Unless it's a fairly large specimen or a small tank, it won't suck out a *huge* amount. Also, periodically, the "leaves" will turn white and fall off, returning calcium to the tank in the form of a super fine sand. In fact, a great deal of tropical beach sand is made this way.
Calcareous algaes such as Halimeda, Mermaid's Fan, Shaving brush, etc. all use calcium. But they also absorb and feed off of phosphates and nitrates as well.
Bottom line, they're cool looking plants, and as long as you keep an eye on your calcium levels, I think they add a lot of character to a reef tank.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef