pk4rney

i opened my coolant reseviour only to find a kind of blue sludge it looks like left over washing powder in your washing machine. ive cleaned it out but theres no mixture on the oil side of things and no funny smells coming from exhaust or smoke, the car did have a coolant sensor code on ecu but scrubed it and it seemed to work. i was hoping it was an old code. could i be wrong, please help (99 uk turbo). p.s my pipes are very rusty and the old owner didnt drive it much could it be build up in the pipes?