Update: I removed my shoal of five speckled corry cats from the tank today, because they chose one of my discus to start EATING! They literally came right up to it and were biting at it's fins! It's tail fin is badly damaged, all the webbing is gone, only a skeleton is left, I added slime coat chemicals in to the tank to try and help... Is there anything else I can do for him? I think I may remove the clown loaches and otos from the tank as well, just to be on the safe side.
In all my years of keeping discus, I have never seen, nor heard of, Cories going after discus, nor their slime coats. But clown loaches, and Otos for sure, (I have had that experience) will do that.
I have kept 4 or 5 different strains of Cories with discus, for many years, and have never seen one get even close to a discus.
Are you sure you saw a Cory biting at a discus' fins ? Was it perhaps a loach ? I truly believe you must be mistaken.