Something is eating my coral.

Not sure what it is. I've had this piece of coral for a week and already half of it's gone. I thought that it was the long spine urchin I had till I got rid of it. What I do have in my tank (75 gallon) is
1 clown fish, blue green chromis, carpenter goby, a sand sifting goby, pencil urncin, a serpent startfish, snails. The only other coral I have is a Frogspawn and it's doing good. Water prams are good just checked them today. Ca is a little low so I've been dosing it to bring it up.