Plain old white vinegar!
Where is it? Inside or outside?
Make a paste of baking soda an vinegar. Use a soft scrub brush to work it in. If you can, buy a denture brush at the dollar store. It really gets in there. Rinse well.
I had this problem and had to use a commercial product that is excellent. I am not sure I can name a product, so I will have to ask the moderators here to allow it first.
Hi Jennifer - This is the shower cleaner that we use to help keep grout and tiles clean and minimize the mildew - it works on kitchen grout too. Hope this helps! Hugs, Holly
PS - We have a FREE eBook with recipes for all of my homemade and all-natural cleaners. You can download it here --> https://pinkfortitude.com/thank#GreenClean