It's just a discoloration on the SS. I experience this discoloration sometimes when I cook a starchy food in a SS pan.
It will come off easily with a light cleaning with Barkeeper's Friend. IF you don't already have BKF, get some. It's the best thing for cleaning off the crud that soap and water can't handle. It's low in abrasiveness so it won't mark the finish on your pans.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan