The growth dynamics of graphene on Ru (0001) and the air stability of this interface have been studied by helium atom scattering. Graphene on Ru (0001) shows high reflectivity for impinging atomic (He) and molecular (H2) beams. The diffraction pattern shows the occurrence of peaks arising from the Moiré pattern. Air exposure reduces He-atom reflectivity, due to water dissociation at the graphene/metal interface and the subsequent formation of CH groups. However, upon annealing at 450 K the reflectivity of pristine graphene has been recovered. The exposure to atomic hydrogen induces a reduction of the reflectivity by more than the 65% with respect to the initial value.