I had green caterpillars eating holes in the leaves of my broccoli and brussell sprouts. Since they are all the same species, perhaps that is what you have? They blended in really well and were tough to spot when they were small, but became more obvious as they grew. I usually found them on the inner leaves. I just picked them off manually and have seen no new caterpillars or leaf damage for weeks.

Looks like it did what cauliflower does -- forms a little head that is now "ricey" and overmature. It might or might not have had a happier ending if you had blanched the head as soon as you saw it starting to form. The plant looks great.

If you can see the green caterpillar then most likely it's the cabbage looper. Sometimes called an inchworm. I just flick them off into a coffee can with some detergent and water. You can use BT which kills them by being systemic. That means it is actually inside the plant and the plant is now deadly to the worm. I've never used it since it MUST have some effect on the rest of the world. I'm probably being a bit paranoid there. Try the detergent first. The damage to your plant, going by the picture, isn't severe as yet. By the way, those cute little white butterflies we see all summer are the culprit. Some people spray a little cheap vanilla extract on the plants and it seems to confuse the butterfly (moth). They go everywhere and apparently recognize your brassicas by aroma. Makes sense. Then they lay their eggs and along comes the hungry caterpillar. Hope all this helps somewhat. Best wishes with your plants.

Yup, I agree with nc-crn, it sure looks like white cabbage butterfly caterpillar damage. Those little green caterpillars really do blend in. If you can't find/smush them all, lol, you can spray the plants with a bacterial slurry (no chemicals) called Bt (Bacillus thurengiensis) which is a type of bacteria that kills only caterpillars. One of several brand names is Thuricide which you can find at most home improvement super stores and good nurseries. Keep us posted. Other than the slight foliage damage, that's a fine looking cauliflower plant! Happy gardening!