Mike;

You better think about getting rid of those flatworms, if they're the red planaria type. They will outcompete other sand bed animals for food, seriously depleting your sand bed diversity, and I feel certain that as their population grows and decreases, they leave a low level toxicity in your tank. Maybe if you get the bryopsis under control, you can concentrate on the flatworms. It takes a few treatments to get 'em all, and I have some suggestions for a specific process; IOW maybe you can learn from my mistakes.