What's growing in my tank? Small threadlike things

[DanG]

They're hard to describe, but it's like multiple pieces of brown thread with small black spots. They seem to be growing from underneath and from within some of my liverock. They're very thin, but some of the bundles have as many as 15 or 20 threads, some reaching upwards of 4 or more inches.
Anyone have any idea what they are? Are they bad? They don't seem to be hurting anything...