What can cause white spots under penis foreskin?

Unclear, but...: The specifics of your Q are not clear enough to deduce reasonable answer, such as referring to smegma, which is the debris of normal skin shedding-off, called, desquamation, or small lesions of <1 mm along corona of glans or beside frenulum, called, pearly penile papules, which are inborn and benign. So, have a urologist take a direct look to assure what you really refer to.