Hard white spot?

[Gary Ellis]

Posted this earlier but can't find it. What is this hard white spot. It is under a rock in a cave and about 2 inches in diameter. Algae does not grow on it.
'.

 

[Reeferdood]

Algae won't grow on it if it is under a rock and being shaded from any lighting. My guess is that it is a sponge of some sort, if so it is a harmless filter feeder.
Keep an eye on it and see what transpires, if anything...

 

[Gary Ellis]

Would a sponge be hard?

 

[vetteguy53081]

+ 1 on sponge and can be firm wall

 

[Gary Ellis]

Will it multiply and how do you get rid of it?