There are numerous possibilities that come to mind.
Either you tracked Flower, investigated Flower, or watched Flower visit someone, but none of those would clarify why you are mentioning the failed kill, unless you somehow know that Flower attempted a kill and failed.
Usopp was definitely killed by the vig, mafia wouldn't do it. So the other failed kill should be scum. And you know for sure that flower is responsible for the failed kill.
Well, I mean Mango saying he knows for sure that Flower didn't visit Sussop might mean that Flower is not the vig at least. Or cause he tracked her... or cause of other reasons.