Since having to rebuild my house since the hurricane I have discovered some stuff called moldstat used by professionals and approved for in hospitals.
Like 26$ for a a 16 ounce bottle that makes up to 40-gallons.
For my specific purpose on wood trusses,Sheetrock,sub flooring it says to spray the surface wait an hour wipe it off and spray again to get down to the mold roots.
A little much for your needs but for what you have I would tear that little bit of matting out find and fix the leak (looks like a firewall hood seal issue to me?) and then replace that that little bit with something more durable and appealing.