Dust. Grime. Bugs. The channels at the bottom of your windows are a trap for all sorts of dirt.
Not to worry though, there is a proper way to clean them.
Some channels can bee pried up, after removing the movable sliding window, and then cleaned.
First you’ll need to spray them with soapy water. Then use a toothbrush to scrub them and loosen the dirt.
Finally, use the clevis tool on a wet/dry vacuum to suck up all the water and debris and then wipe.