What this parameter in the Riemann surface metric has to do with its complex structure? As far as I'm aware, a complex manifold $M$ is a topological manifold together with an atlas ${\cal A}$ of charts $(U_i,\varphi_i)\in{\cal A}$ such that the open sets $U_i$ cover $M$, the maps $\varphi_i: U_i\to \mathbb{C}^n$ are homeomorphisms and whenever $U_i\cap U_j\neq\emptyset$ the transition maps $\varphi_i\circ\varphi_j^{-1}:\mathbb{C}^n\to \mathbb{C}^n$ and $\varphi_j\circ\varphi_i^{-1}:\mathbb{C}^n\to \mathbb{C}^n$ are holomorphic. In that scenario, the atlas ${\cal A}$ is called a complex structure.
Now I'm studying string theory and in the book I'm using (Becker, Becker and Schwarz) what the authors call a complex structure seems like something different. Quoting their book, pages 89 and 90:

Here $\int Dh$ means the sum over all Riemann surfaces $(M,h)$. However, this is a gauge theory, since $S$ is invariant under diffeomorphisms and Weyl transformations. So one should really sum over Riemann surfaces modulo diffeomorphisms and Weyl transformations.
Worldsheet diffeomorphism symmetry allows one to choose a conformally flat worldsheet metric $$h_{\alpha\beta}=e^\psi \delta_{\alpha\beta}.\tag{3.109}$$
When this is done, one must add the Faddeev-Popov ghost fields $b(z)$ and $c(z)$ to the worldsheet theory to represent the relevant Jacobian factors in the path integral. Then the local Weyl symmetry $(h_{\alpha\beta}\to \Lambda h_{\alpha\beta}$) allows one to fix $\psi$ (locally) - say to zero. However, this is not possible globally, due to a topological obstruction: $$\psi=0\Longrightarrow R(h)=0\Longrightarrow \chi(M)=0.\tag{3.110}$$
So, such a choice is only possible for worldsheets that admit a flat metric. Among orientable Riemann surfaces without boundary, the only such case is $n_{\rm h}=1$ (the torus). For ach genus $n_{\rm h}$ there are particular $\psi$s compatible with ${\chi}(M)=2-2n_{\rm h}$ that are allowed. A specific choice of such a $\psi$ corresponds to choosing a complex structure for $M$.

Now I'm puzzled with this last statement. This $\psi$ has to do with the metric, while the complex structure has to do with how we construct an atlas of holomorphic charts and therefore with how holomorphic functions are defined. I can't see how these two things are related.
What I imagined was this: we want to consider all possible Riemann surfaces $(M,h)$. Given any such surface the diffeomorphism symmetry allows the metric to be put in the form (3.109) and therefore be classified by $\psi$. Still, given a certain topology, not all metrics can be defined in the surface with that topology. Therefore we partition the space of such surfaces by genus $g$ and for each genus $g$ we have a class of possible $\psi$.
I'm conjecturing that the same happens with complex structure. In the same way that given a topology not all metrics can be defined in the surface with that topology, given a complex structure not all metrics are compatible with it.
Still I don't know if this is correct, and I think a more complete understanding is necessary here. So why does a specific choice of such a $\psi$ corresponds to choosing a complex structure for $M$?
 A: As I said in the comment, what they wrote is simply nonsense. Here is a couple of references where at least you will not find any obvious nonsense:

*

*As a mathematician, I like the first 6 chapters in

Ji, Lizhen; Looijenga, Eduard J. N., Introduction to moduli spaces of Riemann surfaces and tropical curves, Surveys of Modern Mathematics 14. Somerville, MA: International Press; Beijing: Higher Education Press (ISBN 978-1-57146-353-1/pbk). iv, 221 p. (2017). ZBL1375.14002.
The tropical staff is (probably) irrelevant for you and is discussed in Part II of the book.


*Also take a look at this, which is aimed to discuss integration over moduli spaces and is probably more relevant for your purposes:

Nag, Subhashis, Mathematics in and out of string theory, Kojima, Sadayoshi (ed.) et al., Topology and Teichmüller spaces. Proceedings of the 37th Taniguchi symposium, Katinkulta, Finland, July, 24–28, 1995. Singapore: World Scientific (ISBN 981-02-2686-1/hbk). 187-220 (1996). ZBL1050.32501.
A free version is available here, but there might be some differences with the published version.
