Mould forms particularly where there is an open source of mould somewhere in the room, or where spores can somehow alight on the culture. The worse the conditions provided for the yeasts and bacteria in the Kombucha culture, the greater the chances of mould developing. Perhaps there are mould spores in the tea leaves which you used. Best is the green tea. Green tea is from the same plant as the black tea except that green tea has not been fermented.
Green tea contains less caffeine and a little more tannic acid.
Many health benefits are attributed to the green tea. So try to get green tea in your tea shop. If you start your first Kombucha batch, add the liquid that came with the culture. If you have problems with mould you also may add some spoons of vinegar. The Kombucha loves warmth.