I to had a problem like this, i recently woke up with rust like stains on my palm. I noticed that its actually mostly contained on the top of my skin and not under . It looked like some sort of red powder is stuck in the pores near the stains. Instead of water i tried to wash it pf with my saliva , and soon enough it faded away. What i think is the cause is if u sleep on that arm u may spill some saliva out of your mouth during sleep and during night that saliva oxidase to brown-redish colour you see in the morning . Something like that u can see in dogs mouth , the brown stains mostly seenable in the white dogs