Well... a couple of things need to be said here. First, I don't think that anyone, even at the LHBS, can tell if the white frothy stuff is indeed mold just from your description. Meads often have a kind of white foamy stuff that forms on top as the fermentation finishes, and after a few weeks, that disappears. It isn't mold, and it isn't a problem.
That said, if the stuff stayed clumped together and actually sank to the bottom (not just dissolved back into the liquid), then the campden tablets should take care of it, for a while anyway.
Mold doesn't usually grow in meads, unless proper sanitation procedures aren't followed. It's actually kind of difficult to get mold to grow once most of the honey is fermented out, since there aren't any complex sugars or starches present in finished mead like can be found in beers, so there is very little for the mold to grow on.
Of course, it can happen, so you've done the right thing by sulfiting the must. How far along was the fermentation when this happened? Do you have any SG readings? It's hard to know just by the amount of time elapsed exactly where your mead is in the fermentation process.