Holding church leaders accountable

Can a lay person hold the pastor and leaders of a church accountable for overlooking open sin in the congregation?

Certainly. Following the example of the Christians in Berea (Acts 17:11), Christians have the responsibility of examining the teaching they receive from their spiritual leaders with the message of the Bible. As God instructs the leaders of his church to watch their life and doctrine closely (1 Timothy 4:16), it follows that Christians will speak to those leaders when there are problems with their life and doctrine. When Christians think of having those conversations, they will follow the direction of Matthew 18:15-20. Seeking to restore the wandering is vitally important (James 5:19).

Finally, your question about accountability serves as a reminder that God himself holds pastors accountable for their work (Ezekiel chapters 3 and 34).