Coke in a cup for too long?

So I get Cola in a Styrofoam cup sometimes, in my room/wherever. I found some cups from like a week old (obviously not in suitable drinking condition, that's not my issue), and the remnants in the cup are gross. I can't explain it so I took a picture. Just plain disgusting. It's in multiple cups. What is this? http://imgur.com/rFqCf.png