Vinegar! Plain old vinegar, kills mold spores. Or make a paste with vinegar and baking soda.
try a combo of baking soda and hydrogen peroxide if you don't want to use straight up bleach
I've had issues with mold underneath grout in my shower before. I bought cotton coils and dipped them in a solution of 50:50 bleach & water and left it on the grout over night. Then I used white wine vinegar the next day to scrub the rest of the grout the next day. It seemed to work surprisingly well!