Diesel algae sludge? If it is there is probably water in the tank and more sludge. You should consider cleaning tank and adding a treatment. I see this happen in boats all the time, water in the fuel makes a habitat for algae or bacteria then turns into sludge, it will corrode aluminum tanks form the inside out and start a leak.. I have taken vessels in rough seas only to become disabled as the sludge mixes with clean fuel and clogs filters.