ΔE 29.4, loose: different enough that you should pick on purpose. Light Sea Blue is a little darker, more saturated, cooler than Ignis Fatuus.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Light Sea Blue is a little darker, more saturated, cooler than Ignis Fatuus.
The same difference applies the other way round: Ignis Fatuus is a little lighter, more muted, warmer.
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 Ignis Fatuus on the colour wheel See Light Sea Blue on the colour wheel
All matches: Ignis Fatuus equivalents · Light Sea Blue equivalents