It's possible from a chemistry perspective to get enough apatite (calcium phosphate) precipitated out of the tank water to glue together the substrate, but a more likely candidate is precipitated calcium carbonate from additions of kalkwasser (and possibly kalkwasser slurry).
While it's true that phosphate can precipitate directly from seawater as calcium phosphate, and phosphate can bind to aragonite to form the same mineral, unless the tank was neglected for an incredibly long time (years), or it was massively overfed, there's not enough phosphate around to form enough apatite to be easily visible or to form structural components of the sandbed/rocks.