Answer: I suspect you've found frass (bug poop), indicating that your cauliflower is being eaten by small green caterpillars. Cabbage loopers and imported cabbage worms are two pests that like all members of the cabbage family, including cauliflower. Carefully inspect your plants for these little worms. They're difficult to detect, except by signs of their excrement, because they're the same green color as the leaves of the plants. If you find any, pick them off your plant and destroy them. If there's a huge population you can use Bt (Bacillus Thuriengiensis) to kill them. Bt is a bacterium that acts as a stomach poison, so the critters will continue to feed until the poison takes effect.
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