Salling, who was arrested for possession of child pornography, died by suicide in 2018.
Morris, who played Brittany in the Fox series, told her Twitter followers that, when mourning her co-stars, it is “tough to have to act” like Salling didn’t exist.
In the post, she said she “can’t get through a day without bawling [her] eyes out at the loss of [her] entire Glee family”.
“I don’t feel the need to ever justify something because some things are better left unsaid,” she wrote.
“Y’all who have lots of things to say, I get it, I UNDERSTAND you…somethings are unforgivable. But this holiday season, amongst ALL holiday seasons is INCREDIBLE hard for so many of us. [sic]”
“We did not loose just 2 cast members, we lost 3. And it is SO incredible tough to have to act like that 3rd one is invisible, because even though his actions are unjustifiable, he was a part of our family at one point and he was mentally SICK. Yes pedophelia is a sickness but… [sic]”
She added: “Although I don’t want to have to say all that, its truthful.”
The actor posted the message shortly after Kevin McHale, who played Artie in the series, retweeted stills from one of the show’s Christmas-themed episode. In the images, Salling’s face had been covered up with a vomiting emoji, which Morris branded “offensive”.
Before his death, Salling – who played Puck – had pleaded guilty to the charges against him, but died before he could be sentenced.
Glee aired on FOX for six seasons, from May 2009 to March 2015.