Is this a Coral?

[ChelseaReef]

So a total newbie question, but after staring at the inside of my tank for a few minutes (ok maybe longer!), I noticed this feature (circled in blue) on some recently purchased live rock. Is it a coral, maybe a closed zooanthid? Any ideas? Thanks.

 

[KJ]

Cup coral.

 

[HB AL]

Looks like just a dead coral skeleton.

 

[Shane Sarantis]

dead coral skeleton