You don't state where you live so perhaps you live in a hard water area and this white layer is chalk residue.

Then again it might just be other water purification residues.

Either way it should do no harm, just dig the beds and dig the residues in!

If you are in a hard water area it might be worth while doing a pH test on your soil in case it is over alkaline!

Thanks. Teegee. It is not chalk residue (we used to live in Dover) We are in the Midlands. I have dug it in and it comes back, so I will try a pH test

Possibly mould where the soil is just so damp and cold or could be a form of Michoriza (spelling?)...a benificial fungus which is in all soil and essential for plant life.

If it's the former, good ventilation and hold back on the water a bit may help.

The pH is about 7, according to the chart just about Nautral. Also all my seadlings in trays planted in compost have died. We have reglazed the sides of the greenhouse with twinwall polycarbonade sheets this year. roof is still glass. Could this be a problem? Thanks for help