Removing an unkown stain from my sofa? Help!?

I had a few friends over last night. I made dinner, we had snacks, and were up till late. Well, for the last little while I have been cleaning up the house, and when I got to the sofa noticed a mysterious stain. I have a micro-suede sofa, and usually the stains are cleaned up easily with a damp rag. Not this one! Any ideas on how I should clean this?