you really are gonna want to start with a clean machine, AND keep it that way....
which means, if this is what your seeing on the agitator, think about whats underneath, and worse, under the splash shield, and the outter tub, the scum will be thick....we have seen it all too many times.....get prepared, it will gross you out....
your gonna want to open that machine up, and start taking things apart, filling with HOT water, and start scrubbing...you may have to do this several times until it all gets rinsed away....
this always puzzles me as to how machines get this way, mine never look like that EVER!....nor the fact that your trying to clean clothes in a less than clean machine....and you think the clothes your pulling out are actually CLEAN....
the machine doesn't get that way on its own, and its not the fault of the machine for it to happen....
in any case.....things that you are gonna need, several brushes, including a toothbrush, a plastic scraper, cleaner like 409, PurpleCleaner, bathroom cleaner with BLEACH, and possible rubber gloves....
not judging, just saying
afterwards and prevention.....80% of the wash load should be washed in warm/hot water from now on, COLD water washes kept to a minimum, at least one load a week with BLEACH, and rinse the dispenser out with hot water after daily use....
your machine will last a lot longer this way......sure you can save by washing in COLD water, and keep those savings in a jar to buy new machines, often!