It is most likely a subcutaneous nematode. I have not seen them in dart frogs until yesterday (I recieved a pair of citronellas with one in the female), but use to see them in imported chameleon species. If the worm is out of the skin take a tiny set of forceps and remove the work it. Then save the worm in a ziplock with moist paper and take the worm, frog, and a fecal to the vet (these worm dont always show up in fecals) Look at the skin of the frog for any more as well. Try your best to take the frog to the vet as soon as possible, if they move up the chest cavity they can puncture a lung. Hope this helps..
Mike C
MACS AQUA LANDSCAPE BY DESIGN