As far as invasive species go, the the New Guinea flatworm is among the worst and it’s now living on US mainland soil.

As far as invasive species go, the New Guinea flatworm (Platydemus manokwari) is among the worst. 
Historically, it’s resided primarily in the Pacific region territories and France, but has now made its way onto the American mainland. 
Recently, researchers have found evidence of it living in Florida. 
One of the notable aspects about the flatworm is that it eats snails.
Though many people would prefer not to have snails in their gardens, this type of flatworm consumes them which threatens endemic species.