ΔE 15.6, loose: different enough that you should pick on purpose. Mortarion Green Clear is noticeably darker, more saturated than Ferocious Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Mortarion Green Clear is noticeably darker, more saturated than Ferocious Green.
The same difference applies the other way round: Ferocious Green is noticeably lighter, 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 Ferocious Green on the colour wheel See Mortarion Green Clear on the colour wheel
All matches: Ferocious Green equivalents · Mortarion Green Clear equivalents