Corals and anemones do not have dead blood cells, or bile from a liver, to turn their "poo" brown, like we do. If they discharge undigested material, it will be the same color as it was when they ate it. This is typically things like fish scales, bones, and exoskeletons. We typically don't feed these animals brown stuff, so when they discharge brown stuff, it typically isn't "poo" or undigested food. It's zooxanthellae. These animals constantly regulate the population of zooxanthellae within their tissues. If the population gets a little to large, they must discharge a portion. When they discharge large quantities, we see it as brown looking poo, but it isn't. It's zooxanthellae. If a seemingly healthy coral is discharging brown stuff on a regular basis, it's a sign that something is wrong. This can be caused by photo periods that are to long, or nutrient levels that are to high. For these animals, energy conservation means everything. The more energy they are forced to use on zooxanthellae regulation, the less energy they have for things like growth, self defence/immune systems, and reproduction. This makes it important to create an environment for these animals where they are not constantly discharging large quantities of zooxanthellae, or brown stuff.