Anyone know what this is?

[asylumdown]

Do hydroids form calcareous skeletons? These guys have very fragile little bony skeletons that the green polyp part retracts in to.

And I did a thorough inspection, they're in 3 other places, so they very well might be spreading...

[Starry]

colonial hydroid. can be a pest, and hard to get rid of as they are rooted in the rock and may just go around any covering you put over it. try the torch method,and fry those nasty aptasia while yer at it.
also heard some angels will eat them.