Certified GKH Gardening Expert
The pictures don't show as clearly as I would like, but it looks to be perlite being exposed in the soil. Watering can remove a layer of peat, revealing some perlite. Feeling the texture, and inspecting closer will help you to get a better idea of what it is.
If it is not gritty, like the texture of perlite should be, then it could be a fungus. It would be very hard to identify a fungus from a single photo like this, though.