What should one do if semen is discharged from him—even slightly—while he is praying in congregation, and he is in the first rak'ah and the first row, and the mosque is so crowded that it is difficult for him to exit?

It is not permissible to continue praying if semen is discharged. The worshipper must leave the prayer, holding his nose to conceal his condition. There is no sin upon him for passing in front of the congregants, and he must perform ghusl due to the discharge of semen.