They aren't famous, and they didn't do it on Twitter of Facebook.
Here is what I could find of Alyysa Milano’s statements related to Nick Sandmann and the other Covington kids.
“The man that boy is harassing in the video is Nathan and he’s a veteran.”
“The red MAGA hat is the new white hood.“
“Without white boys being able to empathize with other people, humanity will
continue to destroy itself.”
"Let's not forget," she wrote. "This entire event happened because a group of boys went on a school-sanctioned trip to protest against a woman's right to her own body and reproductive healthcare. It is not debatable that bigotry was at play from the start."
The closest thing I see to libel is the first statement, where she said the young guy (“boy”) was “harassing” the Native American. I don’t thing “harassing” is a statement of fact, or an attempt to state a fact. It is an opinion. Plenty of people would agree that the kid was harassing the Native American; most including myself, would say the Native American was harassing the kid. Also at issue was whether the kid was blocking the way of the Native American. He didn’t move, so many would say he is blocking the way. It was also evident that the Native Americans marched right up and confronted the kids. The other statements about MAGA hat is the new white hood, etc. are just statements of silly opinion, and again are not purporting to be statements of fact, therefore, not libelous.