Midfielder Moder, who was eventually substituted, received a yellow card in the 67th minute after he lunged in and caught Williams on the ankle. The incident was checked by VAR but Moder was not sent off.
Following the defeat, Forest sit in 17th place, three points clear of the relegation zone with 10 games to go.
Last weekend, Forest's referee analyst said match referee Paul Tierney's decision not to return the ball to Forest for a restart, minutes before Liverpool's 99th-minute winner, was wrong.
"Again we are going to send a letter to them and they apologise," Nuno told Sky Sports. "Someone tell me what is happening. It is week after week. Last week it cost us a point at Newcastle. All these situations. It is enough.
"We will keep on fighting. We will regroup. We will analyse the game and prepare for the Luton game. It will be tough but we will be ready.
"I am just asking in a very honest way, tell me why. Why always us?"
