بسم الله الرحمن الرحيم
(Fatwa: 16/206/SN=04/1440)
Semen is thick fluid and somewhat cloudy white in colour. If it gets smeared with clothes etc it can be felt by seeing or touching. The drops which you saw after bath, if you are sure that it was semen then the portion of the chuddar with which it is smeared shall be declared impure and it is mandatory to wash it; Salah will not be valid without washing it. Put the impure part under the tap and rub with hand and flow water until you are sure that the impurity is removed, Thus the chuddar will turn pure. You may also wash the chuddar in tub etc in a way that you wash it thrice with new water every time and wring each time. See: Dur Mukhtar with Shami 1/342, pub. Beirut, Ahsanul Fatawa 2/57, pub. Karachi).
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband