Snowflakes growing in my tanks.

[DNoakes]

Hi everyone,
I have these snowflake like things growing on my tank glass. They started shortly after introducing a couple hundred pounds of live rock. They are currently about 2-3mm across and have perfectly uniform tenticles reaching out from the center. I have a picture of it and am attaching it to this post. In the picture due to the size and the focus ability of my camera the tenticles are not visible but the bulk of the body and shape are visible.
Does anyone know what it is and if it is a good or bad thing to have in my tank? I am running mainly fish only but have also some soft coral and anemonaes.
Any help or advice would be appreciated.
thank you,
DNoakes.