I'm using foot fungus powder on my eggs, applied with a makeup brush. I'm having a lot of problems with mold this year. At first I thought it was the water, so I changed all my eggs to new vermiculite, but now I'm still having problems, so maybe it's the new vermiculite? (argh!). In any case most of the treated eggs are looking GREAT (I had a whole clump of four eggs that was white and fuzzy)... some aren't looking good, but they may have never been viable in the first place.
Also the COLOR of the mold is a big important piece of information. If it is white, you fight it. IF it is a beautiful teal green, chances are you might have lost the egg. I had three eggs turn blue-green... two died so far, and one is well separated from the rest and treated like all the rest, but I do not think it is alive. However the white fuzzy ones give every appearance of being fully alive.
By the end of July all my statistics for this year's snakes should be in and I'll feel like I'm an expert on mold on eggs. Until then I can only hope I'm doing the right thing. Next year, if I can figure out how to do it, I'm going the no-substrate way.