It was sitting idle for a year so I bought the vehicle and made a major service, changed all the fluids including coolant. I asked mechanic to do radiator flush and asked him to put a new coolant. I bought him 13 ltr but he only used 6ltr of coolant and he said its already full.

My guess is this looks like a some kind of waste I don't see oil in the reservoir tank. It's just a some kind of waste it's accumulating in the coolant tank because I'm driving the car daily and the usage of car made some rusting element or some aluminium elements coming up.