mold experiment

I'm doing a science fair project on what foods mold the fastest. One of the food I'm using Is cheese & I have a question about it. I placed the cheese in a zip lock bag on a napkin that was slightly wet. About 1-2 days after, I look at the cheese and it was a bit melted & then I looked at the napkin which was a green color. I'm wondering why did the napkin turned green?