Mould is usually green, blue, black, or white. Some people worry that the brown spots that can form on top of a scoby, or the brown strands that float underneath it, are mould. However, this is a natural form of the yeast colonies in the scoby.
Mould is usually fuzzy. A healthy scoby will be smooth and moist, though it may have some dry patches if you haven’t kept enough liquid in your scoby hotel, or if the air in your kitchen is very dry.
Mould usually forms circles or rings. The filaments in the mould spores tend to grow out evenly from the centre.
Mould is usually on the top of the culture. Unlike the yeasts and bacteria in the scoby, which can work in both an aerobic (with oxygen) and an anaerobic (without oxygen) environment, mould spores need oxygen. In general, things hanging down from the bottom of the scoby, or at the bottom of the kombucha liquid, will not be mould.