They build Sclerites whick are almost pure Calcium carbonate.
The density of Calcium is much less but if you've grown a healthy Toadstool you probably noticed that they put on mass at an incredibly fast rate. SPS have a much higher concentration of Calcium but don't grow at nearly the rate of a Toadstool. A lot of tanks have dozens of SPS so the Calcification rate would be greater than a single Sarcophyton but a tank full of Sarcophyton would be a huge Calcium sink.