What To Do About My Cardinal ...

If you've only had them a week, you still have at least a week to go in quarantine. I'd wait another couple of days. This one will either get better or worse. And the others could join him or stay as they are. When you know which option is happening, that's the time to take action.

What is the decor situation like in your QT, if any?  How extreme is the "hiding away"?  If it's very extreme, then I'd be on the lookout for any other behaviour changes over the next week or so, but, if it isn't too extreme and there aren't any other behaviour changes, it's probably not much to worry about.

When I got my x-ray tetras 9+ months ago, one of the smallest two had a tendency to "take time out" from the crazy chasing that the others were doing (presumably establishing their place in the hierarchy) and from playing in the filter current with them - he seemed to enjoy staring at his own reflection in a separate section of the tank. However, over time, their behaviour has changed with different ones deciding they like being on their own (one female had a tendency to hide in the cave ornament for ages after water changes) or pairing/grouping off differently.

When I put my harlequins in the QT 2+ months ago, the smallest one started breaking off from the other 5 and occasionally being chased by some of the others. He hid for some time behind a fairly large ornament in the tank; eventually, I removed that ornament and replaced it with a silk plant which meant that he didn't develop the habit of hiding away too much as I feared may have been the "downfall" of the first of the two pygmy cories which died. Since they've all been in the main tank, he is still the smallest, and the one most likely to sleep highest up in the tank above a plant or be last to get food (or lose it from his mouth in the excitement of getting some) or occasionally be chased by another harlequin or even to shoal with the similar-sized pygmy cories; however, none of these is so extreme that it's concerning me eg he's not hiding inside the cave-like ornament and doesn't seem perturbed. I've been ensuring that post- water changes and re-instating the furniture (ornaments and silk plants), there is nowhere so "cut off" from the remainder of the rest of the tank that any fish could become too isolated and a recluse.  I appreciate that if you've got real plants, though, then there may be less scope to rearrange the decor to prevent this sort of situation.

Hope that insight to my experiences is helpful/reassuring - and hope it all works out ok.