I don't think it matters too much that your soil is free draining as Liverwort will also grow on just about any surface of soil that is damp, especially those that have been fertilized for plants, as they prefer it.

I have a constant battle with this as the tops of my pots always have a layer of this damn thing on the top. I just scoop it off every now and again and replace it with a top dressing of soil.

Sprinkling some sharp sand on the surface seems to help, but I have found that this only works for the plants that I have in the greenhouse or conservatory that I water. The one's that I have outside are susceptible to rain and not under my control.

Hope this helps Fred?