Salam alaikum, in the net at famous sites of heretics, you would see such claim against companions:
When Ali said to Anas: “Why don’t you stand up and testify what you heard from the Messenger of Allah on the day of Ghadir?” He answered, “O Amir al-Mumineen! I have grown old and do not remember.” Thereupon Ali said: “May Allah mark you with a white spot (of leprosy; Alphosis) unconcealable with your turban, if you are intentionally withholding the truth.” And before Anas got up from his place he bore a large white spot on his face, Thereafter Anas used to say, “I am under the curse of the righteous servant of Allah.”
They gave as a reference 3 books.
This athar isn’t saheeh or hasan, but it is weak.
First of all we don’t know where did these misguided quoted this version from! Because neither in Hilliyatul Awliyah, nor in Musnad of Imam Ahmad, doesn’t stated that this person which was cursed by Ali, was Anas. In Hilliyatul Awliya it comes in the volume 5/pages 26-27, without naming Anas, and in Musnad (#964. thk: Arnawut) where stated that 3 person didn’t say that they remember such words, and Ali prayed against them. And again no names.
Second. Chain in Hilliya weak due to uncertainty in narrator which suppose to hear this from Ali – Umeyrat ibn Sad al-Hamadani. Yahya al-Qattan noticed that this man wasn’t from those who relied upon. (see Mizanul itidal 3/298).
In Musnad this hadith come in the additions of Abdullah, and chain is also weak due to uncertainty of Walid ibn Uqba and Simak ibn Ubayd.
Thirdly, even in Maaref of ibn Qutaiba stated that author said regarding this report that it has no base.
InshAllah would be updated.