Mildew inside shower handle

I have a standard delta single shower handle. Inside of the plastic handle mildew is forming. I saw this about 2 years ago and replace the entire assembly.

Soon after I replaced the assembly, I began to see black mildew forming. I still haven’t done anything about it since then. Now it’s getting more and more black.

I thought replacing the cartridge would fix it. Any advice. I know I should not have let it go this long.

Mildew likes cold dark damp places....like a bathroom! The main way to avoid mildew in the bathroom is to improve the ventilation. That said, the inside of acrylic knobs is a problem area. Even if you don't have a stem leak, it will be a damp environment, and I think mold likes the plastic surface. I used to take of my PP Windsor handles twice a year to clean the inside, but bleach and other cleaners are harsh on the appearance, and I just gave up and replace the handles about once a year now!

You can also throw them in the dishwasher.
I knew one lady that put her dinning room light bulbs through the dishwasher too.

Thanks. I wasn't sure if there was a water leak or something that was pushing water up inside the handle.

If all I need to do is take it off and clean it that's great. I was wondering if I need to repalce the part that the handle screws into....