What flag can fly above the United States (U.S.) American flag?

Yes, there are actually two flags that can be placed above the U.S. American Flag.

The Church Pennant during church services for the personnel of the Navy and No other flag may be flown above The United States flag except at the United Nations Headquarters. The UN flag may be placed above flags of all member nations.

§ 7. Position and manner of display

(c) No other flag or pennant should be placed above or, if on the same level, to the right of the flag of the United States of America, except during church services conducted by naval chaplains at sea, when the church pennant may be flown above the flag during church services for the personnel of the Navy. No person shall display the flag of the United Nations or any other national or international flag equal, above, or in a position of superior prominence or honor to, or in place of, the flag of the United States at any place within the United States or any Territory or possession thereof: Provided, That nothing in this section shall make unlawful the continuance of the practice heretofore followed of displaying the flag of the United Nations in a position of superior prominence or honor, and other national flags in positions of equal prominence or honor, with that of the flag of the United States at the headquarters of the United Nations.