in theory, mold should spread and will infect other cuttings near by. the spores are easily disturbed and what not. but funny thing i have noticed is, the mold are localized and will not move to other cuttings. maybe it's because i catch them fast enough with daily check.
anyway.. if it's inside of the cup, water it with diluted hydrogen peroxide. or uppot to 1 gal and while doing it, get rid of the part of the soil that has the mold.
i had mold inside of one of the cups. since i use peat pot method, i just took it out, clean out the side of the peat pot with mold, spray H2O2, and repot. it never came back. but now i make sure i have holes on the side of the cup for whatever little circulation it requires.