A grey-green iris reads as more muted and desaturated than a vivid green, sitting closer to the grey category with a green cast rather than a strong, saturated green.
'Grey-green' isn't a formally separate category in most classification systems — it describes a real, common in-between appearance that sits on the boundary between grey and green as those categories are usually defined.
This is one of the pairings most affected by white balance — the same eyes can photograph as clearly green in one image and grey in another.
No photograph or online colour-picker tool can definitively classify which of these categories a specific pair of eyes belongs to — classification is inherently somewhat subjective, and lighting/camera effects change the apparent result.