Scale or something else?

[Roberta]

Where I have used the coffee grounds trick, I have not had any problem with mold. (But then, I'm growing outside... so with light and air exchange, it's an environment not conducive to mold) If it got moldy (regular snail bait does) I would just brush it off. When I have done it, I really don't use much - more like a few good pinches than a handful. But brewed coffee would certainly eliminate the possibility of the "residue" growing stuff.