Does masturbating cause spots to appear under the foreskin ?

They are more likely to be sebaceous glands that release a kind of oil to keep your glans penis moist. If you don't wash your penis properly this mixture of oil and dead skin causes smegma or "cock cheese" as repulsive as that sounds. Nothing to worry about at all. And no masturbation does not cause spots anywhere, neither does it make you go blind or give you hairy palms.