Markings/smudges/wear on steel springform pans
Looks like normal markings on well used baking tins. It's fat that polymerised from heat (same procedure as for seasoning cast iron, just unintentionally). It's perfectly safe to use the pan like that. It can theoretically be removed with outrageous amounts of elbow grease and heavy duty cleaning chemicals, but it's not worth the effort in my opinion.