It could be a wart, or a fungal infection, dermatitis, eczema, lichen planus, or a long list of other possibilities. The only way to know, unfortunately, is to see a doctor. You may need a biopsy to know for sure. A wart doesn't typically rub off in the shower, so go get this checked soon. It's not likely serious, but if it's a fungal infection, you could transmit that during sex, or it could spread, so you'll want to treat it. Don't attempt to treat it yourself - you could make it a lot worse if you use the wrong thing.