Anemones don't have dead blood cells, or bile from a liver to turn their poo a dark color. When they poo, they simply discharge portions of a meal that weren't fully dissolved, so their poo will be the same color as the food they eat. When they discharge dark colored stuff, typically brown, this is usually zooxanthellae. Anemones will do this throughout their lives, but typically this is in such small quantities that we don't even notice. When they discharge large quantities, it could be a sign of trouble. They will do this when nutrients in the system are to high, causing the zooxanthellae inside them to reproduce at a rapid pace. They will also do this when the lighting is changed. Then there is general stress like temperature that can cause them to bleach, and we may see them discharging large amounts of zooxanthellae as they do. Does any of this sound like something your anemone could be going through?