That's the A/C dryer. The end cap is correct, but there is an "O" ring that is probably leaking. The green stuff is the oil that's used in the A/C system.
Get a shop that can do proper A/C repairs, or the dealer, to check the system out.
Thanks! Glad to know at least it's not missing part. Is the green oil refrigerant? I heard that fixing A/C system is costly but maybe just replacing an O ring won't be too much?