Oo...new to gardening, so not sure, but I think this was mentioned on gardeners' world last week or the week before - could you watch it on catch up? Something to do with too much condensation I reckon.
x

This is an algae growing on the surface of the compost. You are using a peat based compost - change to a loam based compost. The compost is too moist - reduce watering. Too much sun - unlikely but allow some shade. Give a top-dressing of grit to reduce the light hitting the compost.

I agree with Bulba, it is caused by the soil being too damp and in the sun, which is causing algae to grow. You can mash it into the soil with your finger if you want to make it look less obvious. I get it reasonably often in my seedling trays and in itself it does not damage the plants.

thanks guys.