If it's a hydroid, it stings and I would remove it. They don't grow all plague like though. Once you remove them they should be gone.... pretty much.
If it's not hydroids, it doesn't look like anything beneficial and I would remove it just in case.
I think I would have just plucked that one out.... they sting, how bad for a fish, I'm not sure.
Do you have the original pic there? Uncropped or resized?
Originally Posted by wrxman06 http:///forum/post/3137114
This pic is upsized. The mystery plant is about 1 inch tall and the others are 1/2 inch.
Ohhhh, I was thinking much larger. I agree with Cranberry 100% - Hydroids.
You do realize that releasing information drip by drip (hard, growing, 1", etc.) just makes the ID a lot more difficult.