Don't know about a tool but if there was more of a radius on the bottom side of that glow plug stem's tip, so that it was more ball-like rather that as flat as it appears in the picture that would make the clip easier to expand when the connector is pulled on greater. That sharp flat underside doesn't allow that clip to get out of the way easily. Even a taper would probably do the same thing. Looks like the clip gets fully trapped by the sharp walls and then can only be removed sideways completely to release it's lock-in-place