Can mold get into food or drink items that have never been opened. Also i have a bottle of mouthwash that contains alcohol the expiration date is july 2019 and it has never been opened, however there is a really small particle inside the bottle. I believe that it is some kind of fungus (mold). Maybe one similar to candida which can change its form when it comes in contact with stressors in its environment.
You really, really need to read a book now and then. You know, like when you're in a waiting room and there are magazines available. There are some things that are so well known that people just assume you know them.
Mold can get into a bottle or can before it's sealed. That's why people caution you to throw a can away if the ends swell. But other than that, mold can not enter a sealed container.
Mold can not live in alcohol. Yeast can live in alcohol up to 10% strength: that is how beer and wine get made.
A particle found in mouthwash is almost certainly a bit of the plastic seal off the lid. Spit it out and get on with your life.