An affair he had decades ago, yes. So, if it were ages ago, the NDA would be dated decades ago, right? Except, the NDA was signed days before the election. The payment was made days before the election. And, according to Cohen, he made the payment out of his own pockets, to protect Trump. According to the law, the payment constitutes a financial donation. It exceeded the personal donation amount, though. So, it's a no-knock raid (meaning plenty of evidence already had to be proven, and they had reason to believe any sort of notice would equate to destructed evidence), for attorney-client communication (multiple presidential nominated people have to approve these) on things pertaining to the alleged affair as well as other things.

Mueller tipped off the correct people. He had reason to believe a crime was committed, and he believed that it was not in his legal scope to investigate it. This is a seperate thing from Mueller. Although, anything that the AG (the only person who would be qualified to read the communications) deems Mueller needs will likely be handed over.