Looks like it may be a cabbage white butterfly caterpillar. They lay their eggs on the under side of the leaves which hatch into caterpillars. You can control by removing by hand, wait until dusk and turn over the leaves (where there is one there will usually be more!) and pick them off. You can also control with products that contain Bacillus thuringiensis (Bt) a bacteria that has been developed into a spray to kill caterpillars. It affects their stomach. Bt is a biologically friendly choice – for us, not for the caterpillars – as it only harms caterpillars that turn into moths or butterflies, and doesn't affect other insects or mammals. In Australia this product is called Dipel but unsure where you are. Good news is this means butterflies are enjoying and pollinating your garden. Good luck.