I recently heard an argument somewhere about why do many Africans Americans believe in Christianity even though it was imposed on them by slave owners years ago. Is this an example of the genetic fallacy?

Some slave owners also imposed education on slaves, so should they reject education because of that? That is just an example of a analogy that can be used to see that the original statement is probably fallacious, although it does not tell you why.

When someone makes a statement such as the one in your example, there is usually an implied (or implicit) claim being made as well. Before we scream "fallacy," it is usually a good idea to rephrase the statement and make the implicit claim explicit. For example, "Are you suggesting that the truth of Christianity is somehow affected by the fact that slave owners imposed the beliefs on slaves?" Of course, that is a fallacious claim and would most likely be countered by something such as, "No, but it does explain, in part at least, why so many African Americans believe in Christianity today, especially in the South," which would be a reasonable claim.

I would agree with you in that the argument, "slave owners imposed Christianity on African Americans many years ago, therefore African Americans should not believe it" might best be characterized as committing the Genetic Fallacy .