This is the first sign that your grow kit started its fruiting stage. You will see white fluffy, mold-looking, 'clouds' that will start to pop up on the top of the substrate. They look like this:
Good white mould
If this is happening to your kit everything is going just as planned. From these little spots, the mushrooms will start popping out soon!
Is your kit not growing? Click here and read more about what to do when you're kit is not growing!
Bad white mould
There is also a different kind of white mold that looks similar but this one will take over your kit in a bad way.
This one looks like this:
If the second picture looks more like your kit please read Trichoderma Tarzianum.