Answer to Parasite Case of the Week 577: Planarian worm, Bipalium species, commonly known as the "hammerhead worm". This is not a human parasite. Upon further questions, the patient mentioned that she was taking a nap outdoors, thus explaining the finding of this interesting flatworm on her arm.
There is a lot of great discussion in the comment section on this case, including some recommended reading. HP recommends this open access article in BioInvasions Records, which includes images of both gross and histologic features, both of which are consistent with this case. The University of Florida also has this nice post on land planarians which are invasive species in Florida. Thanks again to Dr. Feely for this cool case!