Leaky skimmer line -- skimmer plugged/capped -- need to remove plug/cap

We have a leak in the line from one of our skimmers. When the leak was discovered it was not repaired immediately. Instead, the skimmer line was plugged or capped where it reaches the bottom of the skimmer.

I want to fix the leak, and I need to run water through the line to find the location of the leak.

How do I remove it? Do I pull or twist, or what?

I don't want to do any damage.

I would appreciate any help. Thank you.