What is this in my reef tank?

[onlytoby]

I'm currently going through the ugly phase but I've noticed this very white substance on my rock.
Does anybody have any idea what it is?

 

[lapin]

Looks like sponge

 

[vetteguy53081]

+1, poriferan sponge to be precise. If it starts to overtake the structure, expose it to air or Inject with vinegar and it will stop spreading

 

[lapin]

It will grow over bubble algae and smother it. Bonus !!!!

 

[onlytoby]

Ooo awesome, thanks guys! I love how in saltwater tanks some new lifeform will just grow out of nowhere!!!