The incident happened in the 15th minute of Saturday's match at Twickenham, which England won 25-21.
Marler has already apologised to Wales prop Samson Lee for calling him a "Gypsy boy" in the same game.
The 25-year-old forward, who plays his club rugby with Premiership side Harlequins, faces an independent disciplinary committee later this week.
Former England centre Jeremy Guscott said Marler needed to be punished for his "Gypsy Boy" comment and said it would be "inappropriate" if his half-time apology was deemed sufficient.
Joseph Jones, a spokesperson for the Gypsy Council, told BBC Radio Wales that Marler had "put himself in a bad position".
"When comments are overheard by other people, even if the person saying them isn't saying them with pure racism at heart, it's going to be interpreted that way," said Jones. "He's put himself in a bad position."
Prop Joe Marler has been cited for allegedly striking an opponent in England's Six Nations win over Wales.