If you zoom in and take a look at some of the striation marks running through the suspect stuff:
... you can see that something was able to scratch through it easily, probably flatware or a knife. I personally use the same chainmail scrubbers that I use on cast iron to clean that sort of stuff off of stainless. It won't leave 'tracks' like steel wool / Brillo might.
Whether or not it's mold would be up to a field test to confirm, but I highly suspect it's just some muck / soot from a pan, you generally don't see mold settle in sinks outside of ring seals around them or the drain.
Some hot water and a good scrubbing should be all you need.