Accretion rate is the sum of all particle mass flow hitting a surface over the surface area. Do not get confused with particle related data (streams, parcels, particles related information) with Eulerian/Continious carrier phase information.
Now for the mdot. For steady flow simulations it is just the current flow rates of the particle streams per number of stochastic tries as they hit the face (kg/s).
Whereas for transient flow simulations it is the sum of the masses of the parcels hitting the face (kg).