Try applying a baking soda/water paste on an old toothbrush and lightly brush the affected area.
Five Stains Baking Soda Can Remove Around the House

Stains, stains, go away — but you don't have to breathe ammonia to get rid of them. A little inexpensive baking soda and water will take out a wide variety of stains and save a lot of expensive repairs.
Crayon marks on walls
Water stains on wooden floors
These can be removed with a sponge dampened in a solution of baking soda dissolved in water.
Stains on porcelain sinks, toilets and plastics
These can be removed by applying a layer of baking soda and then using a damp sponge. This also works well on a water stain in a sink that couldn't be removed without heavy duty chemicals and scratching the surface.
Coffee mug stains
Remove stains from coffee mugs by wiping the mug with a wet cloth, then rubbing the inside of the mug with baking soda. If the stains are stubborn, soak overnight in hot water and baking soda.
Crockpot cleaner
Soak the crockpot in warm water and baking soda before scrubbing for easier cleanup. Or make a paste with water, apply it to the stain and let it set for a few hours, then spritz with water and scrub the paste to remove the stains. This also helps remove sauce stains.
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