This is a discussion on I.D. help within the Coral and Reef Creatures forums, part of the Advanced Saltwater Discussion category; -->
I first noticed it when it was around 1/8 of an inch and has since grown to about 3/8 inch. It has a white ...
unfortunately I can't I.D. it for you, but I am very interested to find out as well... I've got several of them growing in my tank, except that they aren't growing in the main display. There are several growing in the overflow corner of the tank (my overflow is in the back left corner), behind the overflow wall so I can only see them when I am behind the tank looking at it.
Since they haven't moved into the main display or the sump (and I don't have a camera and wouldn't know how to informatively describe them), I haven't bothered to find out what they are. But now that you've asked and provided a picture, I wanna know as well!