Brake*
Where is this pic? It looks like this might be a front wheel up near where the line attaches to the upper control arm?
Hard to tell by just that pic. Does it taste like brake fluid? jk lol, but did you touch it with your fingers to see if it's oil, washer fluid coolant, etc?
I believe that's the ABS speed sensor wire, not a brake line. The residue is likely from something else.