It is not poop. No one feeds their anemone chocolate. Anemones do not have blood cells, or bile from a liver to turn their poop brown. If they discharge any undigested material, it will be the color of what was eaten. We don't feed dark brown stuff like chocolate, so any discharged "poop" will not be brown. What anemones do discharge, that is brown, is their zooxanthellae.
There had to be a reduction in color. It may not have been noticed, but it had to take place. The anemone is basically a semitransparent bag of water. All of that dark brown material was once inside the anemone, so the anemone had to be darker when this material was inside it. Once all that color was discharged, the anemone had to have faded in color. Sometimes this isn't all that noticeable. This is why I always take photos of anemones I'm trying to acclimate to a new system. Any change in color is much more noticeable when you can compare photos taken a day or two apart.
Do you have a thermometer on the tank? If so, what's the temp?