I would clean engine if dirty then check for oil, unless you can see it leak at balancer.
If leaking at balancer you will need new seal & possibly a repair kit if their is a grove at seal location. You won't know this until you get it a part. The changing of the seal is easy getting to it looks like work as you will need a puller to remove balancer. Also need to make room in front balancer to get it out. Working on diesels requires patience, I am in the middle of 2005 E450 box truck. (upper engine upgrade/repair).