So.. I can't imagine I'm the first person to deal with this.
My ST33FR has three cable-driven slides. All three have the same thing going on right next to the cable mounting points on the upper and lower outside corners of the slides.
Is this something to worry about? It appears to be growing. Is there a fix? If so, what is it?
This is not water caused as the camper has only been in a handful of rain storms and is dry as a bone. I store it in covered storage.
Here are some pics.. hopefully, this will help.