mildew on soil

You may have the soil too wet, which will cause your plants to damp off.
What is it that you are growing?.
You may need to water with a solution of cheshunt compound.
When sowing large seeds a dressing of silver sand on top of the soil helps.

Hi Jeano, i sometimes get this in my cold frame and greenhouse, too, could be algy (not sure if spelt right) to be honest i have'nt lost anything due to this yet, damping off would be obvious as the base of the stem rots and the seedlings fall over, as Dr.bob has said over wet soil can cause this, but just because you have algy does'nt mean the stems will rot. lack of air circulation, sun and over moist compost are the cause, i would advise remove it from the soil surface add grit on the top of your soil, and improve air circulation, reduce watering. if however you are growing your seeds in a shady location, could be moss, again you just need to remove it and same as above, hope this helps.