A couple of things that occurred to me overnight. They might have been noted in the many posts at Arms Control Wonk, but they're worth repeating.
For centrifuge enrichment, the levels of moly hexafluoride in the uranium hexafluoride feed probably have to be a lot less than for gaseous diffusion. That means that steps beyond the mixer-settler separations are probably necessary. I am guessing that parts per million are too much. As the gas moves from one centrifuge to another and the separation proceeds, the moly (being lighter than the uranium) will go into the fractions enriched in U-235, which are the important ones.
It's possible that the moly is coming from the apparatus itself as well as from the ore. This problem would be much harder to solve, particularly when it is combined with the requirement for very low levels of impurities. Most steels have some moly in them.