I would putting scrubbing bubble or some other cleaner, let it sit, then wipe it down. Once cleaned and thoroughly dried out, I would seal the crevice so water doesn’t pool.
I would wrap a thin cloth over a fine tool eg small screwdriver or anything that is hard and thin. Then I would use a mold cleaner usually they contain bleach or hydrogen peroxide. And swipe it across with some pressure over and over. The dirt should come out. Best to try and keep the area as dry as much as possible. Maybe place a small fan nearby for better ventilation. Less bacteria that way too.