What is this cotton-like stuff growing on my pizza sauce? The sauce was to expire in Nov 2013. I kept it in the fridge all the time. The thing that grew on it was white in color (just like cotton). There was a greenish center. The overall "ball" was about 1 cm in diameter, and they were all over in the bottle. 

I don't have a picture of it, because I had to throw it out. I don't know what it was, or how it started growing.

What was it? Why was it there?
It's mould. You've probably seen it growing on bread. Refrigeration slows it down, but eventually it'll grow.

You didn't say when you bought it and opened it. Perhaps your fridge is not set cold enough?