Crystalline looking growth - sponge or hydroid?

RockStacker

Reefing newb
This white crystalline looking thing has been growing on the base of one of my sarcophytons.
I already yanked it out one time with my aquarium grippers and a piece of the base "tube" remained and was too wedged in to get with the grippers.
A few days later, this thing has grown again to its original size.

I did not touch it with my hands, but through the grippers it felt rubbery rather than calcified.

Any ideas what it is? is it some sort of sponge? or a hydroid?

DSC_0014-5.jpg
 
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