Many anemones will get a good amout of their food through photosynthesis if you have enough light. For a sebae it more than likely would require a good amount of Metal halide lighting (which is the most intense style of lighting) to survive if you were not feeding it some type of meaty food a couple of times a week (mysis, silversides, brine, flake). If it is white it has expelled all of it's zooxanthelle (internal algae) which it creates as a food source during photosysnthesis. An anemone can survive with less than required lighting for a period of time (sometimes even long term though this isn't recomended) if it is getting supplemental feedings of meaty foods containing protien. If you haven't been feeding and you don't have Metal halides unfortunately it has probably consumed itself to the point of starvation is not going to make it. Your best bet is to pull the anemone out an smell it. If it smells really foul remove it from your tank it is done. If it doesn't smell bad and it isn't completely defalated try feeding it finely chopped raw shrimp or some mysis shrimp. Like I said it doesn't sound good if you are seeing the white stuff (it's internals coming out of it but I have seen some pretty ssick anemones recover). Good luck!