We study a popular kinetic model introduced by Saintillan and Shelley for the dynamics of suspensions of active elongated particles. This model, involving the distribution in space and orientation of the particles, is known to exhibit phase transitions. We focus on the linear analysis of incoherence, that is on the linearized equation around the uniform distribution, in the regime of parameters corresponding to spectral (neutral) stability. We show that in the absence of rotational diffusion, the suspension experiences a mixing phenomenon similar to Landau damping, and we provide optimal pointwise in time decay rates in weak topology. We show that this phenomenon persists for small rotational diffusion, up to time ν^(−1/2). The interesting feature of the model is that the usual velocity variable of kinetic models is replaced by an orientation variable on the sphere. The associated orientation mixing leads to limited algebraic decay for macroscopic quantities. To prove such decay, we combine several ingredients, starting from general pointwise decay results for Volterra equations that may be of independent interest. While, in the non-diffusive case, explicit formulas on the sphere and stationary phase arguments allow to conclude the desired decay, much more work is required in the diffusive case: we combine there an optimized hypocoercive approach with the vector field method. One main point in this context is to identify good commuting vector fields for the advection-diffusion operator on the sphere. Our results in this direction may be useful to other models in collective dynamics, where an orientation variable is involved.