If a player covers his mouth and says something, and this has a racist consequence, then he has to be sent off, obviously. There must be a presumption that he has said something he shouldn't have said, otherwise he wouldn't have had to cover his mouth. If you do not have something to hide, you don't hide your mouth when you say something.
The International Football Association Board (IFAB), which sets the laws of football, said it will consult about mouth-covering, after Benfica midfielder Gianluca Prestianni was alleged to have racially abused Real Madrid winger Vinícius Júnior during a Champions League match last week while his mouth was covered by his shirt.