Hi Amanda,
Thank you for taking the time to reach out and share this lid concern with us! Our slider lids are designed to close tightly to prevent as many spills as possible, but that can also cause them to retain liquid underneath the slider. For that very reason, the slider is designed to be removable for thorough cleaning. You can view the video on our Care Instructions to see how to remove the slider lid for cleaning. A link to this can be found at the bottom of any of our web pages under the Support column.
We also recommend rinsing or dishwashing the lids immediately after each use, especially when drinking thicker beverages like smoothies, or darker beverages like coffee.
Best,
The Swig Life Team