What’s growing on the side of my pipe organ???!

[hexcolor reef]

After cleaning my nano tank after upgrading from IO to Red Sea coral pro salt (A1 stuff) I was scrubbing algae off a couple coral and seen a white skeleton growing on the underside of the pipe organ. Not sure if it’s an Acro???

 

[homer1475]

I forget their name, but that is not a free acro.

 

[Tcook]

Could be bleached out halimeda algae.