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That looks like cyno bacteria to me.. Which has nothing to do with the anemone but is a water quality issue,to many nutrients like nitrate and phosphate..what is your water parameters and your feeding regime and maintaince routine?
Most likely waste, which means you are over feeding it. What do you feed it and how often?
As far as that being normal, I would say no. I've had my sebae anemone for almost 10 yrs now, never directly fed it and it never expelled waste like that.