are you sure Mariob...That is highly zoomed in pic for zoanthids...lighting, aparture and other things could effect the color you are seeing as green...
Take another look at the pic. At the very top of the LR you can see the long tentacles showing against the blue background of the tank. Even my button polyps don't have tentacles that long.
you're right, they do have long tentacles. guess what threw me was the green color.
now that most agree they are hydroids, should something be done to control them.
FWIW, this tanks (26g bf) is only to maintain the live rock & a few cleaning critters until I get my 110g up & running. LR will be used to seed additional base rock.
I wouldn't be worried about them. In most tanks, they seem to vanish after a while. I rarely see them lasting for very long. However, if they do happen to like your tank (I think they require pretty heavy feeding really) they could become problematic to control.