(28 old male) Penile problems

I noticed a patch on the corpus cavernosum of my penis, right at the base—underneath, between the scrotal sac and the beginning of the shaft. I just woke up one day and saw what looked like white powder. When I washed it with water, it disappeared easily, so I didn’t think much of it. It doesn’t itch, it doesn’t hurt, and it felt more like dry or flaky skin.

Over time, that area started looking lighter than the rest of the penis, but it’s only at the base. The white powdery stuff seems to show up every 20 days or so. The skin that flaked off seems drier than the rest—less oily and maybe even a little dehydrated.

I’m wondering: is this something serious? Should I be concerned? It doesn’t cause pain, itching, or discomfort. It just shows up occasionally, and every time I wash it, it goes away. But that spot has stayed lighter and feels drier than the surrounding area.

Could this possibly be some kind of STD?

I started noticing this spot on my penis on April 5th, the morning after I had a casual sexual encounter. I used a condom, so it was a protected encounter.

I don’t think it’s an STD, because the spot appeared the very next day—do STDs even show up that fast?

I’ve already done rapid tests for HIV, syphilis, and hepatitis, and all came back negative.