Three possibilities: Biological growth,
Diesel fuel precipitate, and fuel contamination from a fuel
purchase.
My guess biological growth. The bugs grow in the zone between
water in the bottom of the tank and the
diesel fuel. Water can get in the tank from
condensation from the air. As the tank warms in the day and cools at night air is exchanged through the vent. As the tank cools the moisture condenses and sinks to the bottom. This happens a lot in colder climates. Best preventive is to keep the tank full - especially in winter.
When Diesel sits for a very long time sludge can precipitate from it. This is different than bio growth. Quality of fuel can cause this but you as a consumer have no way to tell if your diesel will precipitate sludge.
Contaminated fuel is always a possibility. Your tank may have been clean but that fuel you bought was in a few tanks between the refinery and you. The fuel
dock should have a filter in their
pump to trap this but maybe not. Best way to avoid this is to buy fuel from someone that sells a lot of fuel.
I'll bet that when you clean the bowl that stuff will be soft and slimy. If so it is biological or sludge. If it is hard and coarse then it got pumped into your tank.
The good news is that your
Racor is doing it's job and protecting your
engine. Once it is cleaned out keep a close watch on it. Diesel fuel stabilizer may help. Putting a hose down to the lowest point in the tank and pumping out fuel until it runs clear may be a good idea. If it keeps coming back then you need to get your tank cleaned which is another topic. A couple of tanks of fuel may just flush it out.