can anyone tell me what this is?

[tkdman]

found this white looking stuff on one of my rock. Kinda looks like the hairs on a bristle worm or a stinging caterpillar.

 

[Neoalchemist]

Hydriods, they can become a problem. They sting.
You might try to scrape them off the rock with your siphon hose on your next water change.

 

[Neoalchemist]

Don't panic though they dont usually do well unless you are feeding filter feeders constantly and tend to die out in modern reef tanks. If your feeding plankton or small particulate filter food then maybe cut back on that. Although, if your feeding a feather star or sea fans or something like that you would probably have to just live with them.

 

[tkdman]

Hmmm can they sting people?
Little bit ago i got stung by something in the tank and my arm swelled up had to take benadryl.
Could never figure out exactly what it was.