You can soak it in vinegar and baking soda and then thoroughly clean and sterilize. I forgot about a backup to my backup pump parts once and there was mold. I soaked it in vinegar and baking soda then cleaned with Dawn and soaked in hot water with Dawn. Then I used the Dapple pump parts vinegar spray and let that soak for a little. Then I rinsed and washed with Dapple pump parts soap and sterilized.
My son left a sippy cup of milk at work in August, I found it s few days later and it was colorful. The only reason I kept it is because I had acid and sanitizer that we use to clean the milking parlor. Anything less than that I wouldn't trust, not even bleach. I'd toss it
personally, I'd toss the whole thing