That is a problem man.....this is per Elegance
Anemones don't poo brown stuff unless they eat something brown. They don't have dead blood cells, or bile from a liver to turn their poo brown. Like Chalkoutline said, they only poo what wasn't digested. Like bones, scales, and exoskeletons. The brown stuff that's often discharged from anemones, and coral, is actually zooxanthellae (The algae that lives within them.) This is very common with newly introduced critters. If the anemone came from a low light environment, it's likely to have a large population of zooxanthellae. The anemone can't survive in a brighter lit environment with such a large population. It must discharge a portion. This is what we see as brown nasty looking stuff coming out of the mouth. Many hobbyists simply brush this off as being poo, and don't give it another thought. This can be a big mistake. The animal is stressed and trying to make adjustments to alleviate that stress. If the stress continues, the relationship between the anemone and its zooxanthellae can break down, causing the anemone to discharge all of its zooxanthellae. In other words, it bleaches. It's important for us to determine why its discharging such a large portion of its zooxanthellae and fix the problem. Most of the time, its a simple case of reducing the amount of light the anemone is exposed to.