Hello: You’re right that the aluminum anode is the likely source of the goo. Replace those anodes with magnesium rods. They generate far less corrosion byproduct. If you have decent water pressure and flow, the flushing kits sold elsewhere on this site might be useful. I hope you have check valves installed on the recirc lines. Spring checks are better than swing checks and will prevent sediment from leaving the tank via the recirc line.