It could be that the temperature is too high. Ideally, the initial fermentation should take place at about 72 fahrenheit. So there are a couple of things to try. First, shorten the time your veggies are sitting out at "room" temperature. As soon as they start to bubble, put them in cold storage. Or else, set the veggies out at 70 fahrenheit, if you can arrange that.
But is the problem that mold is forming on the top of the container? That's really not a problem, usually. Unless the stuff smells awful, just scoop the moldy part off (I know, it took me a while to get used to that idea, but the mold hasn't hurt the rest of the stuff). Keeping the cabbage below the water surface usually prevents the mold from forming.