We have been noticing black specks that leave a greasy trail when smeared in our bathroom sinks and on dishes washed in the dishwasher in recent weeks. After some research, I find that the 2 main reasons for this are 1) a deteriorating black rubber hose to/from the water heater, and 2) a deteriorating magnesium anode rod in the water heater. I pried open a bit of insulation on the pipes to/from water heater and found that they are either copper or solid plastic pipes. That leaves reason (2). How do I determine if the anode rod in my water heater is magnesium (as opposed to aluminum) without calling a professional? Is it typically written somewhere on the heater? Our heater was replaced in the past 3 years. I'm attaching 2 pictures below showing parts of the top of the heater, one of which seems to be the head of the anode rod (I don't know which one).
Also, are there any other major causes for the black stuff that I should be looking out for? I find that decaying black rubber hoses is another reason, but the hoses to the sinks in our house look like the picture below, which does not seem to be the flexible black rubber hose that decays. Can anyone confirm?
Yet another possible reason I find is the decaying inner lining of the water heater expansion tank. Below is a pic of the label on the expansion tank, which I believe was replaced when the heater was replaced in Dec 2017. Does it looks like the kind that has an inner lining that decays quickly?