A biofilm is a structure, not a stain
There is a difference between dirt lying on a surface and a community built on it. Bacteria that settle on a wet surface do not stay as loose cells: they secrete a matrix of polysaccharides and proteins, and that matrix glues them to the surface and to each other. The result is a slime — a hydrogel, mostly water, with living organisms inside it. That is a biofilm, and it is what the grey film in the door seal actually is.
This matters because the two things behave completely differently under a wash. Dirt is held by weak forces and surfactant lifts it into the water — that is exactly what detergent is designed to do. A biofilm is held by a polymer matrix that surfactant does not dissolve, so the wash runs over it, the matrix stays, and the bacteria inside carry on. Worse: detergent residue is organic matter, and organic matter is food. Run a "drum clean" cycle with powder and you have just fed the thing you were trying to kill.
What does work is mechanical, and then chemical, and then — most importantly — nothing at all. A brush breaks the matrix physically, which is why an old toothbrush in the fold does more than any product on the shelf. Chlorine breaks the polymer apart chemically, but only where it can reach, which is why it has to go on after the scrubbing and not instead of it. And then the surface has to dry: a biofilm needs a continuous film of water, and a day of dry air collapses the matrix faster than a month of cleaning.
The fold is the whole story, and the door is why it is wet
Look at the seal in section. It is not a flat gasket: it is three lips with a pocket between each pair, and that pocket holds water after every wash. The pocket is also warm, dark and full of detergent residue — which is to say it is an incubator with its own food supply. And a closed door keeps it that way. The single most effective change in this entire page is to leave the door ajar and wipe the lip dry after the last load of the day.
Which also explains the pattern people report: it comes back a few weeks after cleaning. It comes back because the conditions were never changed. You removed the growth, the pocket stayed warm and wet, and the survivors — the ones in the deepest part of the fold that the cloth never reached — grew back. Scrub deeper, treat it once, then take away the water and it does not return.
Two other places, and how to tell them apart with your finger
White, dry, chalky crust in the drawer or on the lip is not a biofilm. It is undissolved detergent, which happens with powder in a cold short cycle, or with too much liquid in soft water. It does not smell musty, it smells of soap. The fix is dosage and one hot wash a month, not scrubbing.
Clean seal, clean drawer, but clothes come out sour and the smell peaks during the spin is the outer drum. Water sits between the two drums, and the floor of that space collects a layer of sludge that you cannot reach from the door — the inner drum is perforated and the gap is only a couple of centimetres. A hot empty cycle with no detergent will lift a good deal of it. If it comes back within a fortnight, or if the smell is strongest at the drain rather than the door, the source is downstream — the pump filter, the drain hose, the standpipe — and that is a different job.