It's corrosion in the coolant, a sign of a badly neglected cooling system.
The good thing is that you noticed it mate. It can play havoc with your waterpump, radiator, heater matrix, thermostat and water channels blocking them up with brown sludge.
To cure it you will need to continually flush the cooling system until no trace of that brown shit can be seen. A lengthy process in most cases but very worthwhile doing before issues arise.