The frequency analysis does not calculate the amplitude or extent of a displacement. It only calculates the shape and frequency of the Eigenmodes. Therefore, the unit that you currently see for displacement in the frequency analysis is misleading. It actually should not have any unit at all. This will be changed in the future.
To get values of displacement and stress during vibration, we can carry out a harmonic analysis. In this simulation, you also need to define which load or excitation is causing the vibration. Based on that, SimScale provides displacement, strain, and stress values.
More information can be found on the documentation page of frequency analyses and harmonic analyses.