ΔE 15.1, loose: different enough that you should pick on purpose. Moonstone Grey is much more muted than Fenrisian Grey.
Can I use Moonstone Grey instead of Fenrisian Grey?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Moonstone Grey is much more muted than Fenrisian Grey.
Can I use Fenrisian Grey instead of Moonstone Grey?
The same difference applies the other way round: Fenrisian Grey is much more saturated.
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.