That piece is like a large macaroni paste; It is a vent tube so it is open at the bottom. You can go to any auto parts store and get a small tube of JB Weld for plastic repair, use sand paper to gentle clean and rough up the cracked area them cover the crack with the JB Weld. You should be fine. Some people have also trimmed a piece of vinyl floor edging to cover the entire top of the headlight just for looks, but that or a flat piece of silicone laid over the top would help protect it from water.

Use a needle nosed pliers to twist the parking light connection to replace the bulb.