Yeast streaks often form in kombucha. Yeasts are the microorganisms that convert the sugars in your kombucha. These like to cluster together and can look different. Sometimes they form brownish or greenish clusters on or under the scoby (Fig.1). Sometimes the streaks hang like curtains in the glass. Don't worry, you can simply strain the yeasts with a sieve before drinking.
Mould or cream yeast grows only on the surface of the kombucha compared to the yeast piles. Creamy yeast looks like a thin white powdery layer (Fig. 2). This often pulls up a bit at the rim of the glass and breaks into many small individual particles when you try to remove it with a spoon. Creamy yeast is not harmful. Compared to creamy yeast, a new scoby forms a cohesive layer that covers the entire surface.
Mould is recognized by the small round and furry islands floating on the surface of the kombucha. Compared to creamy yeast or yeast piles, mould is always furry or has hairs (Fig. 3). Mold islands can have different colours. Mould is unfortunately a reason to discard the entire batch with the scoby.
Fig. 1: Yeast piles Fig. 2: Creamy yeast
Fig. 3: Mould