Normally, you could jump over to the Help & Support chat room and see if a MOD can help out, without posting names. That way, they can action it that way.
Alternatively, take a screeenshot and open a ticket to report. Right now, I think the ticketing system is still having problems but normally the following link would work:
https://support.chess.com/customer/portal/emails/new
I played a game against a member named [public accusation nat allowed but it has been noted -- MOD] today and, consistent with his jejeune user name, he made a racist remark against me right off the bat. How do I report him/her/it, to the administrator?