1.4 16v BXW engine overheat dash light

Hello

My 2009 roomster with 16v 1.4 petrol had the overheating dash light up today (a very hot day). Restarting the car removed the light. It has never done this before.

When cooled down, I examined the coolant it appeared a like it had lost a little. But more worryingly the coolant header cap had a little fresh motor oil on it - car had an oil change last week. Also on the cap was a little white chalk-like grit on the threads.

Any idea what could it be?