That doesn't look black enough to me for a proper bug infestation, more waxy as Vaughan says, I've had filters clogged solid with black sludge before so definately seen it in the flesh.
Eberspacher heaters have a gauze filter on the inlet side of the pump behind the hose fitting, I think webastos have the same.
The bug in question generally lives at the interface of water and diesel extracting nutrients from the diesel and oxygen from the water, if you have metal fuel tanks there's a good chance of condensation build up in the tank, the sludge is often the dead bugs as the living bugs tend to form mats in the tank.
Marine 16 is good stuff, give it a strong shock dose to kill it all off and a smaller dose when filling to keep it clear, a £12 bottle should give a preventative dose for 2000 litres.