That headline is usually the first comment I hear when a boat owner calls me.
Invariably the sail has suffered from lack of use, left furled on the forestay or sitting under a leaky boom bag.
It doesn't take long, under a month you will start to see the mildew in one of its various forms.
To prevent the mould from starting, you need to air your sails ie go sailing, at least weekly, even a half hour run will do your sails the world of good, if not yourself.
Everyone has an opinion, here are mine on sails and canvas, or whatever else I feel a need to rant about.