Is it a sponge of some description?

[Pred24]

Hi everyone, just wondered what’s this growing under a frag plug is it a sponge or a tunicate? Or something sinister..? Thanks

 

[ReeferHD]

are you talking about the blue stuff? hard to tell in this picture, another pic with a flashlight will help.

 

[Pred24]

the "blue stuff" is exactly the same as the one on the left the white thing with a hole in it...

 

[ReeferHD]

definitely an encrusting sponge, most likely wont harm anything but some types can be invasive, i usually just leave
them alone

 

[Koty]

Sponges. They are essential for coral well-being.