At the hospital where I work, there's a clergyman who comes to nurses station every day and asks if "there are any African Americans in the hospital". Is this legal?

I see nothing wrong with the clergyman asking that question.

No, it is obvious that he only wishes to visit with African Americans. What type of clergy is he?
There are some religious groups ( such as the Church of the Black Madonna here in Atlanta GA that teach that the white man works ( or is) Satan, and that the Black Jesus will come to save the black man from Satan. And that any Black person who helps or marries a white person is a traitor to their race.

So a pastor from a group like that would have course not want to visit with white people.

But while I have a problem with the question, it is not a violation of any type