A few weeks ago I changed the wagon thermostat and flushed out the green coolant from the PO's tenure. I noticed some play in the water pump pulley and slow coolant loss so I replaced the water pump yesterday. The green coolant was clean and the weeks old G-05 was also clean. The revealed block and pump coolant passages were immaculate. The pump impeller however was rusty. The old pump has OE/dealer replcement markings. What should I make of this rust?

Sixto
98 E320s sedan and wagon