Fredo knew of the murder of Johnny Ola and the initial attempt on Roth's life.
He knew Ola was dead/murdered, but he couldn't pin it down on Michael unless Michael had admitted to him that he had given that order.
He also knew of the incident with took place in his brothel between Gearry, Neri and the prostitute.
Which was his brothel and his responsibility. We don't know if Michael himself had actually informed Fredo of that plan.
