In his post, which has since been deleted, the former police officer also repeatedly named the alleged gunman.
Another post Casson made on his personal Facebook page in 2016, prior to being elected to the council, called refugees scum following terror attacks in France.
The comments drew widespread condemnation this week with councillor Dave Macpherson referring to it as a "stain" on the council, while Mayor Andrew King moved to distance himself and the council from the views.
On Tuesday, Casson laid a complaint with the council over a series of public posts and comments made in response to the posts by Macpherson.
The two councillors met Wednesday morning with King, after which Casson withdrew the complaint.
Casson posted an apology on his page, which has now been locked down, before meeting with Waikato Muslim Association president Dr Asad Mohsin and other members of the Muslim community at 2pm today.
Mohsin said some members of the community were angry and upset at the comments.
"For that he unreservedly apologised and he apologised to every member of the community.
"We did have some people who were not very comfortable with this because some of them felt more offended. The intensity of the offence felt was different."
He said those upset showed patience and understanding.
"I hope that James has learned a lesson from this."
Casson said at the meeting he did not seek forgiveness for the comments.
Mohsin, a university professor, said the apology was accepted and ultimately the community did not want to stop anyone coming to the mosque.
He said it was not up to him whether Casson should not seek re-election to the council at local body elections later this year.
Macpherson, who attended the meeting, said the apology was emotional and genuine.