What is this stem like thing growing on my rock?

Please help... No clue what this thing is...

 

Looks like a bit of stray macro algae.

Just tear it out. Macro takes over your tank. Hence their use in refugiums

 

Ok so its nothing bad right?

 

Nope. Beneficial actually. (If it is indeed macroalgae - which it definately looks like to me).

The only bad thing is they can often become invasive in your rock work and be a pain later on to get rid of etc.

 

It does look a lot like some macro algae. I would venture to guess some kind of caulerpa.

 

+1 on the caulerpa and they are very fast growers and will quickly take over the tank, unless you have a tang in the tank, pull it out or you will never get rid of it.