I think Mueller passed the lead along because it was too much of a stretch for his team to handle personally, having no connection to a person who worked on the campaign.
If Mueller found an FEC violation by a person who worked on the campaign, his team would file the charges.
I also speculated that this is a fishing expedition instigated by Mueller, hoping the search will uncover something Mueller can fold into his report.
ping to this comment by Cboldt.
FEC violations are civil, AFAIK.