That fitting is an electrical connector. Pull it off (use a lever if necessary), if there is diesel leaking from that socket, go and buy a new shutoff valve.

Easy DIY job, get a new o-ring at the same time. The valve is held by 2 torx bolts.

Use degreaser to clean the area up.


By the way, commonly the areas around that valve that leak, are the fuel pipes that connect to it, and the o-ring between the valve and the fuel pump. Do check they're all ok first, and that they're not the actual source of the leak. Shutoff valves aren't that cheap.