Guernsey seems to be lacking any native Flatworms, I have not found any yet, where Alderney does have Rhynchodemus sylvaticus.
Not natives can normally be easily separated due to having lots of "eyes" (or light sensitive cells) along the length of the body where natives tend to have only one pair of eyes. It is widely accepted that the invasive species have been imported over time through the horticultural trade, many of the species imported from a long way away wherer they are native in places such as New Zealand and Argentina.
The current Guernsey species list is:
I have added some photographs which I do have below.
If you do come across these, please report them to the Guernsey Biological Record Centre or states department ACLMS so that spread can be monitored, or more likely confirmed that they are probably now found island wide.
FLATWORMS
Australoplana sanguinea
Kontikia andersoni
Kontikia ventrolineata
Obama nungara
Rhynchodemus sylvaticus (native)
PLANARIANS
Argonemertes
c.f. australiensis
Geonemertes
dendyi
Marionfyfea
adventor