Everybody Kahm Down!
It’s not mold, it’s yeast, commonly called Kahm, which is a sort of catch-all term for a variety of yeasts that can form a film, or pellicle, on top of ferments.1 It’s harmless. No one is going to get hurt here. It’s alright, no need to panic. It’s incredibly common. It can, however, negatively affect taste. Our lid system prevents the yeast from coming in contact with the ferment. You can just scrape it off. Covering your ferment with cheesecloth makes it easier to remove.
1- In an early version of this post, I inadvertently named the yeast as Saccharomyces cerevisiae. Thanks to those who pointed it out and to Ben Wolfe for clarifying.