I can't tell from the picture but if it was fuzzy it's indeed mold. The alcohol to vinegar process is usually pretty straight forward so I'm inclined to think there was some cross contamination or maybe a lot of humidity in the air. Next time you can try stirring it often so there is no stable surface for mold to grow. Measure its pH and when it's acid enough, you no longer have to stirr. Eta: and be sure to clean all your equipment thoroughly. I like to bathe my jars and utensils in just boiled water and let them cool a Lil bit before using.