The anemone could have stung the coral. That might account for it.
Also the anemone moving around it might mean that the current in the tank shifted. That happens to me when I clean pumps and such. Since acropora like a good amount of current, that might account for it. Also the pump might have moved to where the coral is getting direct current from it, and that could be another reason as the direct current might be to strong.
Also the stuff almost looks more purple than brown which is usually an abundance of nutrients in the tank; however, your parameters don't seem to indicate that.
Another thing might be your lighting, if your lights are older then it could just be the shift in intensity and spectrum that is causing the issue.
One more thing is to check for red bugs as it also could be that something is munching on it and that is causing the issue.