It's a pest anemone. They're a "glass anemone", they're non-photosynthetic, but can be. Under bright lighting, they'll darken and become brown, under low light, they're transparent. They breed really quickly, especially when disturbed and can be a nightmare to rid a tank of once you've got them.
There's several ways of treating them, from compounds like Aiptasia-X, or "Joe's Juice", to things like a red hot screwdriver that'll boil it on contact, but that requires removing the rock which isn't always possible.
Then there's copperbanded butterflyfish which sometimes eat them, and "Aiptasia eating File Fish" which as their name suggests! :wink: