Maybe the green glow is there for gameplay reasons? Maybe gearbox found out that the spitter attack was too good with a pure black stealthy design.. (unseen snipper running around really fast in the dark..no way that could be overpowered.) ..so they added the green bags to give marine players a way to spot it before getting hit.
Just speculation schmeculation.
by the way this early image of a new xeno is quite similar to the spitter... look at the arms. An early design or a different xeno?