My testicles have 'dandruff' on them, what could this be?

Fungus. This is almost always a fungal infection of the scrotal skin. I ecommend trying an over the counter anti-fungal cream for a few weeks. If that does not help go see a urologist or dermatologist. Patricio gargollo pediatric urology childrens medical center, dallas, texas.
See Derm/urologist . This does not sound like any specific condition but could represent an infection or reaction or inflammatory process. I would meet with a physician for an evaluation.