Wall Faucet in garage

I have wall faucet in my garage, and there is a cap on the faucet, and water is coming out from the cap. I just need to get this cap off, should i use pliers, or should i use screwdriver. Already tried to twist with pliers, but still leaks. How do I get the cap off, the edges are smooth?

Thanks
Jamal86

Please help

sloan

The inside of the tailpiece has corrosion on it at the VB slots. Clean the inside with a 1 1/2" copper tubing brush or sand cloth.