You could have mesh motion normal to the boundary by selecting unspecified in the Geometry definition tab of the wall dynamic mesh zone. This option is only available if the linearly elastic solid mesh smoothing method or the radial basis function smoothing method is enabled.
However, I would still recommend that you either couple your Fluent simulation with Mechanical or set up an intrinsic Fluid-Structure Interaction within Fluent if your deformation is small and remains in the elastic regime.
Chapter 25: Modeling Fluid-Structure Interaction (FSI) Within Fluent (ansys.com)
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