Being a RFID UHF antenna a "passive" element does it need to be FCC certified? Or, we just need to have the RFID UHF reader certified?
A antenna by itself does not radiate, so can't be certified. The entire unit of a intential radiator, including the antenna, absolutely needs to have FCC certification if you want to legally sell it in the United States.
If this antenna is connected to a receiver-only device that never intentionally radiates, then certification is not required. However, it still must meet the radiation limits for a unintentional radiator, but it is up to you to make sure of that. You don't have to get it tested, but if there is a complaint and the FCC finds it violating the limits, you're in deep doodoo.