Stichodactyla haddoni.
It should eventually close its mouth more for you, if it's newly introduced they do sometimes hold their mouth open (makes for better respiration). Prolonged gaping can be a sign of stress but S. haddoni is perhaps one of the more resilient carpets so should be OK.
Two things to watch out for - they are very sticky, anyone who blunders into it usually ends up being a meal. The other thing is they can get kinda large. I eventually had to give mine up because it was close to 24" in diameter and it was giving a new definition to the phrase "wall to wall carpet".
Good luck! So far it looks great. It will be interesting to see how it settles in for you.