The precipitate is likely magnesium hydroxide:
What is that Precipitate in My Reef Aquarium? by Randy Holmes-Farley - Reefkeeping.com
Precipitates When Carbonate Solutions are Added
Many aquarists are familiar with the cloudiness that forms when high pH two-part calcium and alkalinity additive systems are added to marine aquaria.
Figure 3 shows the initial cloud that forms, for example, when the alkalinity part of B-ionic is added to a relatively low flow reef aquarium. The initial cloud sinks and spreads out, eventually dissolving. A similar phenomenon is not observed when adding calcium or magnesium salts, but is observed when adding sodium carbonate solutions.
This cloudiness is, at least in part, magnesium hydroxide and is formed when hydroxide ions are added and the local pH rises. Unlike the addition of limewater, which is unlikely to form magnesium carbonate, this may, although I think it unlikely. The reason it might form here is that the addition of the carbonate ions may push the magnesium carbonate solubility product above saturation. The precipitation of magnesium carbonate can be kinetically slow, just as the precipitation of calcium carbonate can be slow, and since this cloudiness forms instantly, magnesium hydroxide is a much more likely candidate. However, if the additive is not rapidly mixed in, or worse yet, if solid globs of the initial precipitate settle out and are very slow to dissolve, then conditions may be ripe for magnesium carbonate (and calcium carbonate) to form.
In any case, any magnesium carbonate that does form will probably dissolve later as the pH returns to normal reef aquarium levels, so whether the initial cloudiness contains any magnesium carbonate or not is not a critical issue. It does not contain calcium carbonate if mixed in reasonably quickly (a couple of minutes or less), as CaCO3 would not dissolve when mixed with seawater (and this material is observed to dissolve).
