Brown grime in Coolant reservoir

[dinfow]

In my 2013 A5 TFSI I noticed that my coolant reservoir was below the minimum, bought some G13 from the local Audi dealer and distilled water ready to top up. Today I took the reservoir cap off and noticed theres a brown grime/sludge around the rim of the reservoir opening, and peering inside it looks a bit brown. Attached some pics to better show this.

I've not topped up the coolant before and only had the car 6 months (bought from Audi dealer), but have heard a mixture of things of what this can be.

Do I need to get it flushed and start again with new G13, or am I ok to just top up with the G13/Distilled water mix?

[Belle427]

Does not look too serious to me but if it were mine id drain the system, remove header tank and clean it, refill with correct coolant/water mix.

[catso]

Had similar on my old A3, 3.2. Caused leaky oil (inter)cooler, causing oil to cross over to the coolant, the amount was small so didn't notice any drop in oil level or increase in coolant, just a brown emulsion.