What is this on my closet wall? How should I clean it?

This looks like damp/mould to me. It should come off with a mould and mildew remover but you’ll want to ensure good ventilation to avoid a return a of the damp.

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Our top recommendations are usually Bar Keepers Friend (great for kitchen surfaces), melamine foam (Magic Erasers), Murphy's Oil Soap (wood cleaner), and Nature's Miracle (enzyme cleaner). Make sure you use cleaners appropriate to the surfaces you are working with and follow all safety labels.

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It looks like it might be nicotine stains leaking through the paint