Is it obligatory to repeat the prayer if a white mucous substance is found after Asr prayer, without having felt its discharge during the prayer time?

The basic principle is that an event is attributed to its nearest time. Your prayer is valid unless there is an indication that the discharge occurred before the prayer, in which case it must be repeated. However, it is more scrupulous to repeat the prayer as a precaution, based on the Prophet's (peace and blessings be upon him) saying: "Leave that which makes you doubt for that which does not make you doubt."