Hi, Conch-lers!

May I ask a really elementery (I would have said "dumb," but I
really don't think I deserve that label) question?    I was just cleaning the calcereous deposit off a Tridacna squamosa, and
it occurred to me, what causes this deposit?   It's not coral, in this case it's not marine worm tubes, or barnacles, just that white deposit that we all clean off numerous shells.  What would cause calcium to precipitate onto the surface of a shell?

Thanks to anyone who will take the time to answer this.    I have collected shells for thirty-five years, and it never occurred to me to ask this question.

Linda