If the U Joint is spinning in the yoke, that's not good. Could be the cap is seized, but you'd have a pretty bad vibration first. Is there any angle on this joint in operation?
It shouldn't be spinning in a complete circle, just through the operation angles - if it's spinning.
However, if it is spinning, I would replace it before it ruins the yoke. My guess is someone played with the clip before to try to repack it and had it going in circles. I have done this before with driveshaft u joints to reuse them.