When $\Gamma$ is nonelementary, I think the condition is simply that the map is the identity on $\mathbb{R}$. These $\mu$ are the Teichmuller trivial differentials forlimit set of $\Gamma$.
This should be in Gardiner's "Teichmuller Theory and Quadratic Differentials" somewhere in the chapter on the Teichmuller space of a fuchsian group.