Looking closer your image, which is unfortunately quite low res, reveals that your background has plenty of noise or real detail which has green hue and the same contrast as the lost hair => no treshold is ok. The background detail also has the same size variety than the lost hair => no treshold nor spatial frequency separation will work. 

Assuming that the hi-res version doesn't reveal more difference, your options are:

1. For future shots improve the light, it must knock out the green from the background everywhere on the target without killing the greenness of the background (bring the background further, increase the white light on the target, be sure that the background has uniform light, the distance helps to keep it blurry and suppress background details, its good light prevents noise), take very rapidly (=automatically) another shot with possibly artistically impossible lights  which gives good mask and use it with the acceptable lighted photo

2.  A theoretical approach, needs some advanced math software: Pattern matching software can calculate which hair is the most probable reason for the existing image . No idea, if that software is available for something else than forensic investigations, high level security systems and military intelligence.

**What to do now:** Your hi-res image CAN reveal more. Insert a link to one. Then some of us with some real knowledge beyond my guesses can find something useful.