If brown stuff is coming out of its mouth, it's not "pooping". It's releasing zooxanthellae. If it's a new anemone, it may simply be adjusting to your lights. You will want to keep an eye on its color to make sure it isn't bleaching. If you have strong lighting, you may want to add a few pieces of window screen over the tank to shade the anemone. Then remove one piece a week to give the anemone time to adjust. If this is an established anemone, and it is releasing brown stuff on a regular basis, the nutrient level in the tank is to high.