How to know if corals are dying.

Hey guys, i have 3 corals in my tank right now, i have zoas, a flowerpot coral(which hasnt opened in a few days), and this coral(pic attached). I wondering when u know corals arent doing well. I know the zoas are doing well cuz they open right when the lights come on. I am also getting green algae on rocks and glass as well.



heres the pic

http://ift.tt/1z68tSJ