Depending on what the anemone is eating, waste can be brownish or reddish. I have only read about lighter, being a result of a diet made up of silversides.
They can turn to self-cannibalism if they need food and don't have it, so a change in the colour of waste should be considered in regards to it's general diet.
I forgot to aclimatice a LTA to an overpowered LED fixture, and it immediately let go of a whitish cloud, and it lost a little colour (it is recovering nicely), but that was through a few tentacles.
When they die, the brownish cloud they release before starting decomposition also comes out through the tentacles, so I think it is normal waste functions you see and nothing to worry about.
If they had any higher abilities, I would claim anemones try to make people worry on purpose.