That looks like a plate coral.
BTW my clownfish live in my branching Frogspawn(Euphylia Paradivisia) witout any problems, and i know someone on the forum whose clownfish live in a simillar coral than yours.
Hi Warr - Definitely a HelioFungia..... or: Long-tentacled Plate Coral.....
Don't worry - those corals won't "eat" your nemo's - only nemmies could do that - IF the nemo's were sick - then they "lose" their chemical protection - then nemmie feasts..... Corals - no....