Are you sure it was mold? You can look at pictures of molds vs. SCOBY. I have been fermenting for 3 days minimum for nearly 6 months and have never had a batch go bad.I’m relatively new to feed fermenting. I’m using a large plastic food container with holes punched in the lid.
I don’t fully ferment any more although I may try again this winter. In the summer, it was fermented in the morning and mouldy by the afternoon and the feed got wasted.
What I really like about it is there is minimal mess and if done right, minimal waste.
Given the chickens here are free range and eat the most disgusting stuff in the compost I’ll never be able to tell if fermenting makes a health difference.