Cast Iron Skillet / Pan stains
But there is some stains left... do I need to clean this or is this normal and leave it ?


As long as the "stain" is hard and dry rather than gummy or sticky, it's probably fine. As commenters have noted, it looks like polymerized oil. You probably cooked your steak at very high heat with some oil in the pan. If you didn't use oil, the stain might just be from some of the fat that rendered out of your well-marbled ribeye. Either way, some of the fat/oil polymerized on the bottom of the pan due to the high heat. This is exactly what you'd do on purpose if you were to season the pan yourself -- coat it in a (very!) thin layer of oil and then stick the pan in a hot oven for a few hours.

As you keep using your pan, it'll acquire a lot of those stains, and it'll eventually be black all over. That's completely normal — it's what a cast iron pan should look like, and the black seasoning is a good non-stick surface.