The mask was found at a planting in one of the municipality's parks.
This is the latest in a series of finds since the flatworm – which, among other things, feeds on earthworms – was first confirmed in Sweden, in central Malmö last November.
The leaf-flatworm, with the Latin name Obama nungara, has, in addition to Malmö, also been found in, among other places, Blekinge and Dalarna.
It spreads through trade in plants.
"Since the species is not established in Sweden, it is important to check in and around pots of plants being bought this spring and onwards", says Maja-Lisa Jarstig, who is a park engineer in Jönköping municipality.
