Soak it in Excel or peroxide. No need to use bleach and then have to wrry about whether you got it all out. I also gently scrape the ceramic disc with tweezers to remove a very very thin amount off the top. I don't recommend it with expensive diffusors like ADA but I use the cheap ones anyway.
Like I said, I would not scrape the expensive diffusors like ADA but I've been doing it to the cheaper ones for YEARS and have never had a problem. I'm not saying take a belt sander to the thing. Just gently scrape the disc for a couple seconds. I've got some of the cheapo .99 cent diffusors that I've had for years and always do this.