At the moment GRHydro doesn't ensure properly that the atmosphere is set after initial data. This is no problem as long as the ID thorn takes care of this, the ID doesn't contain atmosphere or the EOS is sufficiently robust to 'recover'. However, this is not always the case. Thus, I propose to set the atmosphere mask in PostInitial, initializing it according to whatever was setup there by some other thorn.
The patch to do that is attached. It is only a change to schedule.ccl as the corresponding function already exists.