Is congregational prayer valid if the imam remembers the presence of semen on his garment after rising from the Tashahhud, and must he inform the congregants to repeat the prayer?

The madhy (pre-seminal fluid) is impure and nullifies ablution. It was obligatory to leave the prayer immediately upon remembering its impurity. The prayer of the imam who completed the prayer after remembering the impurity is invalid, and he must repeat it with a pure garment. As for the prayer of the congregants behind him, it is valid according to many scholars, because the presence of the madhy was something hidden from them. Some scholars overlook a small amount of madhy on the garment, and according to this opinion, the prayer is valid.