it?s s little tough to say but either it?s dying or throwing out it?s guts (maybe it ate to much/something irritating it) how long has it been like that ? kinda looks like a bubble tip. what?s your water chemistry like?
Unfortunately it is dying and most likely won't recover. Tank looks fairly new is it? Anemones need an established (stable) tank, good lighting and flow.
I think it was just attacked by something, throwing out the stinging innards as defense. much like when you poke at ricordia, or hairy mushrooms too much.
long shot, but any chance it looks like maybe you're only looking at half of the anemone? When they split then pretty much just stretch themselves over the rock until they tear into 2. Sometimes after mine had split, they would shrivel up and go under a rock for a few days.