Could be a mold of some kind, it'll happen when there's not a lot of fresh air exchange and the soil wet, usually it's not much to worry about, I've seen mold on the surface of the soil in my pots that was white and even green, it never really seemed to affect my plants. If you're really worried about it just increase your fresh air exchange by opening up your terrarium more often and fanning it, or you could simply cut a few holes in your clear plastic lid so it'll have some exchange naturally all day.