What has grown on top of my potting mix?

[elight]

Hi all,

I mixed all my potting mix and placed it in my containers last week, but it wasn't warm enough to transplant my seedlings yet. So I covered the tops of the containers with plastic bags and left them outside. The potting mix was thoroughly soaked at the time. I came back a few days later and found the following white stuff on top of the potting mix:


Anyone have any idea what it might be? Can I remove it? Leave it? I'd love to know before I put plants into these pots!

Thanks!