The state of the impeller reminds me of when a radiator is added to a loop without having water run through it to clean the inside and then you get those shavings inside the loop from inside the radiator. I've seen those slowly act as an abrasive on pumps before and start to whittle down the pumps impeller.

Sure looks nasty inside, this is one reason I go for transparent acrylic blocks on my graphics cards (EK 480 Nickel) and Transparent Pump Tops (XSPC DCC+ Top). I can clearly see what's going on with regards to algae.