Say I leave a cup of tea untouched for a week, it will have those cotton looking fluffs of mold. But even on the second day the I notice transparent looking stains on the surface, like oil on water, is that the beginning of mold!?
Technically it is mildew (mildew is white, mold is black, green, or blue. People just use those interchangeably, and to even refer to stuff like the red yeast you may find in dirty bathrooms...), but you still shouldn't waste tea, nor should you encourage mildew to grow in places you live in (both mold and mildew are a health hazard)
Tea contains oils etc. So, when it cools down, it slowly starts to separate into hydrophobic and hydrophilic substances and seed oils solidify on surface.
That can be tested easily. Flush everything with boiled water, prepare the tea and seal hermetically.
Oh, so that layer on top of the tea was mildew? Ai-chan thought it was just tea powder separating from the water because tea powder is lighter. Once back in school, Ai-chan swiped some tea into a clear plastic bottle, brought it home and forgot about it. Then about 2 weeks later, there was a brown layer on top of the water in the bottle and the tea became almost transparent. Never thought it was mold, because the bottle was sealed.
Someone ^^ explained it's oil from tea that breaks off after cooling, but there is also mildew. Closing a bottle does nothing, you didn't vacuum it and hermitically closed it. Keeping it in a fridge should prevent mildew, though it is still possible to grow but unlikely. Only in freezer would it be impossible.