the only thing that comes to mind, for me, is b.linens. It is often used to ripen certain types of cheeses (like a muenster, for instance) and is also often found running wild in nature. It may be contamination from the muslin, or from your hands.
If I understand how it works it is commonly found on the human body and is not something to worry about although I have never seen it on feta. I believe that feta should normally be too salty to harbor it.