If the kombucha culture has been weakened, a whitish, wrinkled coating may form on the scoby, called creamy yeast. If the scoby has sunk, this layer can also spread on the surface of the liquid.
This is a mixture of different yeasts that colonize the surface of the kombucha, but are not otherwise a normal part of the kombucha flora. They are called Hansenula, Pichia and Debaryomyce, for example. They usually show up as a wrinkled white layer on the scoby or as small chalky islets that float around on the liquid and quickly disintegrate when stirred.
The good news is that while cream yeast doesn't cause anyone to jump for joy, it is fortunately not harmful or dangerous. It is even used specifically in cheese production. In other ferments such as sauerkraut, water kefir or beer, it is known as an undesirable but harmless germ that mainly disturbs the appearance.