In Motherless Brooklyn, Lionel's boss and mentor Minna is killed because he knew a secret. We later find out that Minna and Billy, the owner of the Harlem jazz club, were planning on blackmailing Moses with this secret, and that's why both got killed. Later,
Lionel meets Moses, who reveals that he raped Laura's mother, one of his employees; Paul signed the birth certificate instead, but exposure of this secret threatened Moses. Lionel warns Moses to leave Laura alone or he will release the information.
Maybe I'm missing something, but all the evidence is just a birth certificate. There is no rape proof. Why would this threaten Moses in anyway? Is it because it's the 1950s and inter-racial relations are taboo? Or would it just cause a scandal, even without proof?