Cabbage worm. The eggs are laid by the cute but small cabbage butterfly. BT will kill the larvae and save the heads.

The Imported Cabbageworm is produced by a small white butterfly while a similar looking pest, the cabbage looper, is produced by a grayish colored moth. The larva of both will be green but the cabbage looper will crawl by bunching its body, like the inch worm.
If there are few hand squishing works quite well and yellow sticky traps are reported to aid in trapping the adults which can reduce the number of eggs laid. If large numbers are present spraying with Bacillus thuringiensis - Kurstaki can help or if absolutely necessary a pyrethrin dust could be used, carefully.

so if I dont see the cabbageworm, im stuck...will this ruin my cabbage? What is BT - and where do i get it? Is it organic/ natural or not?
Thanks......

BT = bacillus thuringiensis. You can get it at just about any gardening center. Thuricide is one name it is commonly marketed under. Read the list of active ingredients -- what you're looking for is bacillus thrudingiensis (sometimes it will go on to say 'kurstaki strain' but not always). It is organic -- and very effective.

Dipel is another brand. BT comes in dust, wetable powder and liquid. I use the dust, one bag lasts me for years.