Plate Coral Question

[TAZ]

Morning all,

Over the weekend I picked up a Plate Coral (long tentacles). For the first couple days everything has looked good. This morning when I looked at the tank, a section of this coral has really inflated a lot (at least 2 times bigger than normal). As it is only in one section of this coral, I have no idea what is going on. I was wondering if anyone else has ever seen this? Is this normal for this coral, or is there something wrong with it? I did a round of water tests this morning and everything looks great.

Any help would be greatly appreciated.

Thanks
Martin