Doesn't 'kras' mean dirt or residue? What does it mean here? "Neg la pa kras we l."..mesi.

yes kras mean dirt or residue as in:
Foubi kontwa a pou w ka retire kras yo.
Scrub the counter to remove the dirt.

But kras also means small amount, little as in:
Mwen pa pè yon ti kras.
I am not scare a little
I'm not the least bit scared

or

ti kras pa ti kras
little by little

so
Nèg la pa kras wè l.
The man doesn't see it a little.
The man didn't see it at all.

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words