Strange thing on my live rock

[stardrop]

 Just noticed this strange green shelf-like thing on my rock last night.  It's about the size of a dime right now, and I have no idea what it is or if it's beneficial/harmful.

Anyone have any ideas??

[groan]

Some sort of plating coral or sponge is my guess. Anyone know for sure?

[Julie]

green plating coralline algae

[rockworm]

Looks like it may be Halimeda algae.  Seems a little soon for plating coraline.  Very common in marine tanks.  Uses nitrates and phosphates to grow, the same as other algaes.  It also uses calcium as part of its structure.  You can get more info here: http://reefkeeping.com/issues/2004-04/nftt/index.php

[stardrop]

Thanks for the replies.  More stuff to research 

[groan]

Cool!