I dont know myself what "grass" texture Afcad use's but its probably an old r8 texture that probably go's back to FS98 days found in the FS9/ texture folder, im guessing you could pic a grass texture you like'd , rename it the same as the one Afcad use's and overwrite it ( saving the original just in case). ive done that myself when getting older sim scenery to work.
cheers ian