Is the white substance that comes out of the husband after urinating impure and does it nullify ablution, and is it permissible for him to perform ablution when the time for prayer begins, even if it continues to be discharged?

Wadi (prostatic fluid) is impure (najis) and nullifies ablution (wudu). The husband must take precaution and wait until its discharge stops. Then, he should perform istinja (washing the private parts), make wudu, and pray. It is not permissible to pray while wadi is being discharged, as long as it stops for a sufficient period to perform purification and prayer, even if he misses congregational prayer. Purification takes precedence over congregational prayer.