Light white substance under coolant cap

[Sambecky1989]

Hi all

Checked the oil and water again today onmy tdi130 passat

Now I checked it couple of weeks ago and I noticed a white substance on reverse of coolant cap
I wiped it off and didn't think anything of it

Checked again today and oil is fine coolant level is fine but the white substance is there again ?????

She uses no oil or water

Temp gauge only sits quarter of way (needs thermostat doing)
But when it's driven harder then normal the temp will rise to 90 then fall back down again

What can this white substance be ?????

Cheers