This is one of the things that people find most troublesome. While these appear to be mold, they are actually something called kahm yeast. This yeast is not harmful, but it can look scary and unpleasant and even smell a little strong. It should be removed from the jar only to prevent it from imparting a strong odor or unpleasant taste to the whole batch. If you can’t remove it all, don’t worry; it won’t hurt you.

Not using the freshest vegetables seems to be one of the fastest ways for kahm yeast to appear. I noticed it most when I used cucumbers and carrots that had been sitting in my fridge for a while.