Would that brownish colour be oil? When the car is cold open the rad cap and look for oil on the inside of the cap and have a good look in the reservoir. I had this problem on my 92 because the engine oil cooler tubes had a slow leak within the rad.
Another thing it may be is that the coolant when mixed properly (50/50) has enough anti-rust properties not to allow the water to rust the inside of the block and heads. If there is too little coolant then you get rust. At this point you are looking at doing a chemical flush which is a bit of a PITA and requires a kit which can be purchased at any parts store. The instructions should be clear enough to do it but if you feel uncomfortable at all (not properly bleeding system of air after could lead to damaging overheating) have a garage do it and it shouldn't be that expensive.
So first step is to identify what that substance is and even test the strength with a tester (which will let you know if the concentration is too low).
1992 Bonneville SSEi
White, grey Leather interior.
Tinted windows and blackouts on headlights.
Custom high flow exhaust, Everything replaceable - DONE
300 000 kms original engine, tranny 2 rebuilds.
Playboy Bunnies Embossed in Tint. Oh Yah!
Ever Seen a Polar Bear in a Snow Storm?