ΔE 19.1, loose: different enough that you should pick on purpose. Medium Grey Blue is a little darker, much more saturated than Moonstone Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Medium Grey Blue is a little darker, much more saturated than Moonstone Grey.
The same difference applies the other way round: Moonstone Grey is a little lighter, much more muted.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Moonstone Grey on the colour wheel See Medium Grey Blue on the colour wheel
All matches: Moonstone Grey equivalents · Medium Grey Blue equivalents