That is the penultimate-ever genuine new coolant tank. Its impossible to clean them tbh, and I would replace them as a matter of course when doing these jobs, but alas I appear to have used them all
The very-last-ever one is of course sitting on my shelf, and bizarrely Tradition don't appear interested in making new ones.
The coolant should be changed every 3-5 years depending on usage. In the V8 it tends to stay clear for a long time as there are very few steel parts in the engine, but in the V6 once the rust inhibitors wear out the coolant will become contaminated with rust and start going brown. It sounds like yours could so with a flush and coolant change.