I recently installed a new faucet into our sink bathroom as well as reinstalled both cold and hot water lines. I realized that there was some water on the bottom of the cabinet. I put a tiny plastic bin and after 8 hours there was only a little bit of water. I checked where it was coming from and it seems to be leaking from above the plastic cap, above the metal nut.
On the hot water line, the white plastic cap/nut covers the entire opening hole for the faucet threads, but it doesn't on the other side where it is leaking. I am pretty sure it's not coming from the metal nut that I tightened correctly.
How do I fix this leak if it is indeed coming from above the plastic cap area and what would cause it? Can I just use some plumbers epoxy (from Ace Hardware) to seal the rest of the opening? I have provided pictures below along with a red circle showing where it's wet ( as soon as I touch this white cap after a while, a couple of drops of water will immediatelly fall down).