If one sees impurity on the imam while he is in prayer, should he inform him?

The questioner : If the imam is seen and there is a trace of impurity on him, or if there is a part of the body that water did not touch during ablution... does the one who sees him have to inform him while he is currently in prayer?

The Shaykh : Yes, if one sees the imam and there is impurity on his garment that is not excused, he must inform him. Likewise, if he sees a spot on his foot that water did not reach, or if he is certain that the imam ate camel meat and did not perform ablution because he forgot and then prayed, he must inform him. This is due to the generality of the saying of the Messenger (peace and blessings be upon him): This applies to the rak'ahs, whether there is an extra rak'ah or a missing one, and likewise to the remaining conditions of the prayer.
The student: ... the prayer

The Shaykh : I understood.

The questioner : Yes

The Shaykh : Yes

The questioner : Now, if the one who saw it did not inform him, do we say his prayer is invalid?

The Shaykh : Is he with him, is he a ma'mum?

The questioner : Yes

The Shaykh : Yes, he must inform him, but how does he do it? He is praying, how does he do it? Even if he recites "And your garments, purify them," the imam may not understand.
The student: ...

The Shaykh : If he recites the saying of Allah the Exalted: the imam may not understand. Does he speak, which would invalidate his prayer, or what does he do? A question came up on this a few nights ago, and we said he should approach the imam, yes, and explain it to him. But the one that came up a few nights ago regarding leaving a prostration, it appears that this is like that: he approaches the imam and pushes him with his hand until he turns away, yes.