if i'm not mistaken..that unknown coral is a hydnophora. if so, it's a very aggressive coral and must be placed away from the others. i had one when i first started my tank..i guess my tank was too new or something, but it died and nuked my tank..after a bunch of water changes, i didn't lose anything else, but everything was stressed to the max
I went to the LFS today and they had more of those corals. It is a form a Monitpora coral, very safe. I looked up the hydnophora and it is nothing like it. Mine is stony
a hydnophora is stony as well. it grows very similar to your picture, except that it is more green. i wasn't sure if it was lighting or camera angle...or just brown polyps.