The green caterpillars that normally eats brussel sprouts are army worms or cabbage loopers.

Army worm (Spodoptera spp): The larvae feeds on leaves, stems and roots of many crops. The caterpillars are dark green in color and always present in mass. Adult grey moth lays eggs in mass on leaves or stem. The newly hatched larvae starts feeding on crown leaves by forming webless perforations. The older larvae cuts the stem and also feeds on leaves by making irregular holes.

Cabbage looper (Trichoplusia ni): The caterpillars are light green in color with yellow stripes running down the back. Also the caterpillar loops as it walks. The adult brownish moths lay eggs on plant debris. The hatched larvae feeds on older leaves by making irregular holes.

Both insects are very active during night.

Management:
1. Keep a close eye on plants regularly. If you find any egg mass on leaves or stem remove and destroy them.
2. Hand pick the larva (better during night) and put them in soap water.
3. Spray homemade insecticide soap solution to repel/kill the insects (add 1 tablespoon of cooking oil+ 2 tablespoons of baking soda+ few drops of Ivory soap in 1 quart of water).
4. Also you can spray biocontrol agents like Bacillus thuringiensis, or spinosad.
5. You can also use commercial traps with floral lures to catch the adults.
6. Keep the garden free from weeds and debris.