>>2774929To me it looks like fond or starch or dairy or something that you didn't properly deglaze. Boil a little water it in and scrape the bottom with a wooden spoon as it boils on the stovetop (put the sponge under the spoon scratchy-side down to cover more surface area at once). Next time, do that with more flavorful liquid (wine, stock, cream, vinegar, cognac, beer, etc.), and use that to make a sauce for whatever you just cooked in it (which is the whole reason to own stainless steel).
Stainless steel is meant for "scratching" and whisking and all kinds of metal-on-metal-contact and you wouldn't be damaging it. Its inherent stickiness at high heat will result in great fond development, which will be ideal for saucemaking. If you don't care about that and just want to get rid of the stuff sticking to your pan, use the SOS pad or comet or whatever. You probably won't scratch it, but even if you do, scratches are totally fine and won't effect cooking at all (and it'll make it look like you actually use them; chicks dig scars)