Are the "flakes" soft and slimey? -- well I guess anything soaked in diesel qualifies as slimy. But algae would be more soft and flexible than say rust or general crud.
My experience with algae is sooner or later you have to deal with it, and it seems sort of like slime clogging the filter. Do as Benzmac suggests, and see if at least some of the flakes in the drained residue sink quickly to the bottom of a glass. If so its not all algae.
Could something have hit the bottom of your tank recently that could knock sediments loose?
Is your tank metal? Rust? If so, I have no idea what you do.
Hope its just algae. The stuff I got from PartsShop knocked mine out.
'75 240D 300000 miles