Assuming those things were not presence in the mouthwash in the first place there are several posible explanations.

• Some leftovers that were released from between the teeth.
• The mouthwash is a blend of chemicals, some can be indicators - change color in the presence of other chemicals or color other chemicals - it can color the leftovers in the mouth in a way that they are noticable.
• in the mix of the mouthwash liquid and the saliva happens what is known as an ion exchange. some ions from the mouthwash "like" to bond with ions from the saliva and make a solid mineral and this will look like small dust parts.