It is quite common to see brine overflowing from a fermentation jar. If you see this happening, then open the container and remove some of the excess food and brine. If your fermentation jar is an open vessel, then place a plate or a bowl under it to catch the overflow. This usually happens because you did not create plenty of headspace between the lid and the brine level.
If you notice that the brine level has gone down instead, then it might be just that your vegetables or fruits have gone up instead. Again, push them back down with a clean fist or a wooden spoon. This will adjust the brine level. If this does not work and only a small amount of water is needed to keep them under, simply add some filtered water to cover the vegetables. If a large amount is needed, then fill the jar up with more brine itself.
Mold is another common problem that people can run into. There are many kinds of mold that grows on foods while they are fermenting. Molds grow inside the jar because it is exposed to oxygen. The best way to prevent molds from developing is to make sure that you are using a good airlock. If you notice a white film on your foods after fermenting, it is fine to eat once you remove the mold from the foods. White sludge gathering at the bottom is also fine as long as it is in small amounts. However, it is not safe to eat the foods if the white sludge is coupled with slime. The creamy film, yeasty odor, pink cabbage, browned cabbage, mold, and slime are all signs of unsafe molds on your veggies. You will have to throw the entire batch away if any of these are spotted.