You are talking about Cardinal Tetras and hopefully not the Red Cardinal, the bird! I'm sure your Discus are not hiding from those teeny little fish. How long have you had the Discus? Have you checked your water for ammonia, nitrite and nitrate? Is the tank placed in such a way that when you approach you are backlit? (That would scare the Discus, too, as all they would see is a dark form approaching). If you recently got the Discus, it could take them quite a while to become comfortable. (Some of mine took at least a month). By the way, I have a small school of 13 Cardinal Tetras and the Discus don't even notice them.
Connie