The awkward title is an attempt at approximating the following specific question: Let $(M^{2n}, J)$ be a complex manifold, suppose $g_0$ is a Riemannian metric $M$ compatible with $J$, and suppose $\phi_t$ is a one parameter family of diffeomorphisms of $M$ generated by a vector field $X$ such that $\phi_t^* g$ is compatible with $J$ for all $t$.  The questions is: must $X$ be a (real part of a) holomorphic vector field in the sense that $L_X J = 0$?

Assume that $g_0$ is Kahler and that $X = \nabla f$ for some smooth function $f$.  As the family $\phi_t^* g_0$ remains compatible with $J$, it follows that $L_{\frac{1}{2} \nabla f} g_0 = \nabla^2 f$ is of type $(1,1)$.  Using the K\"ahler hypothesis one can show that $(\nabla^2 f)^{1,1}(\cdot, J \cdot) = \frac{1}{2} d d^c f$.  On the other hand, since the associated K\"ahler form $\omega$ is closed, it follows from the Cartan formula that $L_{\frac{1}{2} \nabla f} \omega = \frac{1}{2} d d^c f$.  Thus it follows that $L_{\frac{1}{2} \nabla f} \omega = (L_{\frac{1}{2} \nabla f} g)(\cdot, J \cdot) = L_{\frac{1}{2} \nabla f} \omega + g(L_{\frac{1}{2} \nabla f} J\cdot , \cdot)$, and hence $L_{\nabla f} J \equiv 0$, as required.