Does anyone know what this is?

Hello all,

I setup my first sal****er aquarium over the past few weeks and on
Saturday January 17th I added some live rock. One of the pieces had
something growing on it. Both my wife and I liked the way it looked
as it flowed back and forth in the water. However, in the last 4
days, it has tripled in height and we are afraid that it may be
something invasive. Please take a look at the link below and let me
know what it is if you know. (If the link does not work, please cut
and paste into your browser window). If you do, please pass along a
little advice about whether it is good or bad, should I keep it or
remove it, and if I need to remove it, any suggestions would be great.
Another note about this stuff, I noticed yesterday that a piece has
started to grow in my live sand.

Thanks in advance!

Link to photo --- http://www.geocities.com/sal****ersystem/whatsthis.html