Help please. Growing herbs on windowsill and white mold appearing on soil

[Mercythompson]

What am I doing wrong? It's happening on six different pots next to each other.

Pots are on windowsill, above a radiator, soil kept fairly moist.

Is any, or all of that, wrong?

Any ideas why it's happening?

I would think soil is too wet.

Are you sure its mold? I get a white crust on top of my indoor plants but thought it was limescale/salts due to watering exclusively with tap water and the soil not being as disturbed as outdoors.

Ooh, that's happening to my herbs too!