Two house cats suddenly fighting?

Cats recognize each other more by scent than by sight. When Clark came back in he probably smelled differently than before he left the house. He was also seen outside by Mozart, where both your kitties have seen strays "tresping". It's unlikely that he recognized Clark as a friend after this incident. If their fights are as severe as they sound, it is best to separate them and reintroduce them slowly. The two articles below are excellent. They discuss these two types of aggression and how to deal with it in your home.