WhirlOut works. For bad cases, may need to use it 1x/week until you see some progress.
I use this too bc the funk builds up fast out here with our calcium-rich water. I also use a toothbrush and 9% vinegar to scrub the rubber gasket/cowling. No matter how much this kind of maintenance sucks, it’s not as bad as doing one’s laundering by hand.
And this Enviro-Magic green stuff is sometimes hard to find but it works very well. I have removed mildew with it when nothing else worked. I used it for a few decades: http://www.amazonpp.com/mildew-stain-away.html
Caveat: check your septic system and if you need stuff safe for your septic fields read the dang product labels before you buy and use the product.