Have you ever seen a dark edge on a water container? At first it seems so minor it is barely noticeable, most often seen following antibiotic therapy in the water or extended use of KD in the water. Without care, it expands to the lip of the drinker.
A particularly effective way to get rid of this mold is to wash and brush the container with Vanodine. I either pour a small amount in the drinker or in less severe cases I make a solution and brush with that.
I also like to put vanodine in the aviary sink occasionally to help clean out the drain.