I wouldn't worry about it. It should go away.

It's probably the coffee grounds starting the natural process of decomposition- I've noticed that a white fluff starts forming when I store damp UCGs in a container waiting to get enough to throw on the compost pile.

I've seen all kinds of interesting molds when I have a lot of coffee grounds that I pile on the garden instead of strewing around the lawn or putting in the bin. I've never seen it happen with a thin layer or when the grounds are fully composted.

Having said that, I would add that I don't think you should worry.

Mine happens when I leave it in the coffee maker too long. ;-)

Thanks for all the responses. It did indeed go away after the sun hit the area.