It is an S. haddoni. A sandbed dwelling anemone that can easily get over 15 inches across.
They will (( yes, will, not maybe )) eat at least one of your fish -- usually slower moving perching type fish, but they have also eaten angels and tangs.
Prefer slower flow -- just enough to lift up the oral disc.
What size tank do you have?
How deep is your sandbed?
What lights?
Current water parameters -- with numbers please.