Most likely this is frass (poop) from a cabbage looper or other caterpillar pest. It should be fine to eat the cauliflower if you first wash it properly. First, soak the cauliflower head for one to two minutes in cold water, then rinse. Then, use a vegetable brush to scrub the crevices in the cauliflower head, rinse again, and pat dry with a paper towel. After this, you might also try rinsing the head in a solution of 1/2 cup white vinegar to 2 cups water, then rinse with fresh water and pat dry with a paper towel. This can help eliminate some more bacteria.
Most likely this is frass (poop) from a cabbage looper or other caterpillar pest. It should be fine to eat the cauliflower if you first wash it properly. First, soak the cauliflower head for one to two minutes in cold water, then rinse. Then, use a vegetable brush to scrub the crevices in the cauliflower head, rinse again, and pat dry with a paper towel. After this, you might also try rinsing the head in a solution of 1/2 cup white vinegar to 2 cups water, then rinse with fresh water and pat dry with a paper towel. This can help eliminate some more bacteria.
For more information about washing produce, see:
http://www.ext.colostate.edu/pubs/foodnut/09369.html
http://umaine.edu/publications/4336e/