Coffee can stain just about anything it comes into contact with, including plastic. Most plastic is not porous and so is easily cleaned after a single spill.
Stains can eventually accumulate slowly until they become noticeable.
Wash the plastic object to remove any grease or grime that may hold the stain in place. This will help you see just how stubborn the stain is.
Usually regular dish soap works well, or you could run it through a dishwasher cycle as well.
Apply Bleaching Agent
There are several products that can remove coffee stains from plastic. Bar Keepers Friend is a popular cleaner.
Just dampen a few drops to form a paste and spread it over the surface. Or you can mix one part bleach to four parts water and let he plastic soak in it for 30 minutes.
Or you can mix 2 tbsp. OxiClean in 2 cups hot water and soak the plastic for 30 minutes.
Allow the bleaching agent sufficient time to remove the stain and then rinse with a generous amount of water. Wipe it with a soft cloth to remove any residue and then dry it with a towel.