At first glance you might get the impression it's a Favites...or Moon coral...
But the pronounced skelatal "teeth" lead me to believe it's a Montastrea magnistellata...or Boulder coral...
Green center surrounded by a brownish/reddish flesh...?
Here's mine...
I don't have a good close up, but here's a pic from when it was in my Nano...
When I saw your pic, I thought "That looks just like mine did when I first got it..."