if you ever face necrosis on starfish, layman terms means rotting, take it out, use reef strenght iodine and drip on to it till it get better. or use cotton wool with the iodine soak in it and wipe the whitish area.
it might stop, atleast it helps for me. it took me 2 drips on 2 seperate days to save it.
and change to better lights, skimmer and filtration if you ever want to keep anemone healthy.
and a warm welcome to this forum.
happy reefing mate.