Q.Cauliflower Pests
My cauliflower started turning gray at the tips, so I harvested it early. I think it could have gotten bigger, so I cut the gray stuff off the tips when I was doing so. There was this little black bug inside eating my vegetable. Is that what ruined my cauliflower? What can I do to prevent this?

Here is a link with more information and should help you pinpoint the issue.
Neem Oil can be used to treat the plants, but if the Cabbage has all ready been picked you can usually cut out the damaged area.
A Cabbage that is harvested to early may be bitter to taste.
https://www.gardeningknowhow.com/edible/vegetables/cabbage/protecting-your-cabbages-from-cabbage-maggot-cabbageworm-and-cabbage-moth.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/root-maggots.htm