Mold is more likely to form where people smoke in the same room. If mold does form, start with a new culture. Sediment is mostly pure yeast culture, good for speedy start of new batch. Kombucha produces its own preservatives - acetic acid, lactic acid, oxalic acid and alcohol. Periodically remove the bottom layer to rejuenate culture.
Culture gets browner with use.
Let mushroom fall or force it to the bottom to start a new, clean, unconnected culture on top surface. When culture is dying off, liquid has neutral or alkaline pH, tastes sour and smells foul. Mold or non-washable, brown, wrinkled patches may appear that tear easily. Large surface areas and shallow liquid speed propagation and acidification. Do not cover tightly.