What is this orange stain in the bathtub?

I’m subleasing an apartment and the bathtub looks like this - is this bacterial or due to water? Other info I know is that the apartment was built pre-1978 (lead-based paint?). How concerned should I be? I’ll be here for 2 months. What steps can I take to clean it if it’s worrying?

Thanks in advance - happy to answer other questions!