Can I undo this part of the valve?

[quarky]

Hello

I noticed some water in the location indicated on this photo. Is it possible to take that off and look? Do I pop the cap off with a screwdriver? Try and turn the end? Or does it just need replacing?

 

[Doghouse Riley]

Hello

I noticed some water in the location indicated on this photo. Is it possible to take that off and look? Do I pop the cap off with a screwdriver? Try and turn the end? Or does it just need replacing?

That should unscrew if it's not a push on, or has a retaining screw.

The water may be coming from the leaking valve or from either of the two unions. Check both unions are tight, but not overtight.
Once the cap's off you might be able to tighten up the valve with a spanner on the small nut nearest the end of the shaft, but make only a tiny fraction of a turn. If that doesn't cure it, you need a new valve. This will require a partial drain of the system and of course the actual towel rail.