White Fungus - Help Needed

Hi,

I'm new to vegetable growing this year and have a number of plants in my small back garden. In an old paper recycling box I've planted some radishes, small carrots and beetroot. The compost came from a grow bag. Last week I noticed a small patch of white on the soil, that looked a little like a bird dropping. Two days later this white patch had spread. 

The original area of white has turned sand coloured. I've just returned from holiday and the fungus or whatever it is is taking up more than half the surface area of the box!! 

Can anyone tell me what this is and what I should do to treat it (or indeed just leave it?) Any help would be gratefully received!!

Thanks,

H