It might be flea debris. It usually bleeds out pink when water is applied. Try using a flea comb on them and you might snag a flea. If so you will need to get rid of the fleas. There are more fleas in the environment than on the pet. So you will need to get rid of them in both places. I use a small dose of Frontline Spray on my healthy cat for flea prevention. If any of your cats are old or have health issues you might want to treat them differently than your other cats. DERMA-IONX can help with the itching. DEFENDEX shampoo would be helpful in cleaning up the coat and providing relief to the skin. However it is not always easy or safe to bathe certain cats, so recognize your limitations and have a professional do grooming if it would be safer.
Please let us know what you determine.