In an incident that threatens to overshadow rugby league's march into the USA, Leniu was placed on report late in the Broncos' 20-10 loss at Allegiant Stadium.
The NRL have since confirmed they are reviewing the matter, which could lead to Leniu being sent straight to the judiciary on return to Australia if charged.
AAP has been told the claim centres around the use of the word "monkey", with referee Adam Gee calling both captains out before placing Leniu on report.
Gee said he did not hear the slur, but had acted on an official complaint made by Mam on field.
Leniu told Triple M after the match that he was not "worried at all", and that the incident was "just fun and games on the field, that's it".
But Broncos coach Kevin Walters said that Indigenous star Mam was certain a racial slur had been made.
"He's pretty upset in there," Walters said.
"He's adamant (there was a slur), but we'll leave it with the NRL. They're looking at it.
"We certainly support Ezra and our player and everything that's around that."
Broncos captain Adam Reynolds said he personally had not heard the slur, but was shocked by the allegation.
"In this day and age, you wouldn't think this is still out there," Reynolds said.
"I was well away from it. But we support our player. He is pretty upset in there.
"We certainly don't stand for racism and we'll support Ezra and get around him, make sure he's alright."
Sydney Roosters coach Trent Robinson said he had not spoken to Samoan Test representative Leniu about the allegation yet, but said it was important a process was followed.
"He obviously made the complaint, but that doesn't mean that it's right," Robinson said.
"It'll go through the formal process as it should."
The incident came in a brutal but entertaining win for the Roosters to start the season, in front of 40,746 fans at Allegiant Stadium.
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