What the heck is this?

[pbradley0]

Two patches of this stuff have been growing in my new tank for the past few weeks. I haven't removed it because without it my tank is kind of boring. Both patches are attached to the rock and growing straight up about 5 inches. It is very soft and moves with the flow in the tank. It is a light brown color and translucent. Any ideas what it is and if there's any harm in leaving it in the tank?

 

[Michael Hoaster]

Cool! Almost looks like a sponge, but you said it was very soft, so probably not. It looks like the feeding tentacles of rather large worm. I can't rule out plant either. It doesn't look like it wants light in the pic, suggesting a filter feeder.

Whatever it is, it's a beautiful gift courtesy of Mother Nature! Their quick progress suggests your tank has ideal conditions for it.