ΔE 18.8, loose: different enough that you should pick on purpose. Steel Grey is a little lighter, much more saturated than Moonstone Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Steel Grey is a little lighter, much more saturated than Moonstone Grey.
The same difference applies the other way round: Moonstone Grey is a little darker, 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 Steel Grey on the colour wheel
All matches: Moonstone Grey equivalents · Steel Grey equivalents