The pink stain is a non-pathogenic bacterium known as Methylbacterium. Methylbacterium is transported in air and the cells commonly settle on surfaces. When the conditions are right for growth i.e. the right amount of moisture, food, and temperature, a few cells will multiply to many cells, and, since they are pink colored, when enough are present on a surface, the pink stain results.
Consider using a relatively strong household disinfecting solution to attempt to eliminate every last one of the cells, otherwise the remaining cells may grow back to form the pink stain that you just cleaned off of your surface. Mold and fungi may also be transported through air and grow on surfaces in this manner.
( Water Quality FAQs - Champlain Water District )