ID This...

[Webslinger]

This started growing on rock which has been in the tank for 4 years. Is it some kind of spong?

 

[Highgrade]

Looks to be a sponge. Any daylight photos?

 

[Tahoe61]

Nemastoma?

Not my image.

 

[norfolkgarden]

Guessing yes, some type of sponge.

We have a nickel-sized black rock urchin that cruises is over the coralline algae and devours it.
About a day later bright pink sponge colonise the rock.
The sponge is grayish greyish-white in white light and bright pink under actinics for the first week. After that it fades in color but it's still pinkish.

Either that or the flame Angel says thank you for another delicious meal...