Anemone's have only one opening. Thus, they take in food, and release waste through the same opening. When they release waste, they expel all the water, complete with the waste, and shrivel up. Then they inflate by sucking in fresh water.
Anemones do this regularly, so by itself it is not a concern. If it stays like that for more than a day or 2, it is possible it is rejecting the water quality.