Semen on the underpants

Assalamu alaikum,
Respected Mufti Sahab, suppose someone watches a bad movie and small drop of semen appears in the underpants. The drop is not even the size of the hole of a palm or a twenty paise coin. But when he goes to the bathroom and wants to clean it he only puts water on that particular area over the drop,but the water spreads to a bigger area but the drop doesn’t spread,only the water as the cloth is of cotton and so the water spreads therefore he washes that part only and rinses it thrice. So the question is has the underpants become napaak due to the spreading of the water to a larger area while rubbing the dirt or not. Is it ok to read namaz and wazifa and touch and read quran after washing the area or the whole underpants has to be washed or has to be removed and a new one should be worn and then namaz etc can be read.

Jazzak Allah!