Not sure what they are. They do kinda look like colonial hydroids, but the ones that I've seen (in my tank!) don't have that long of a stalk and are a lot smaller. These almost look like some type of clauvaria or anthelia, which are soft corals. (Some will say they're just as bad as hydroids though... since they spread like weeds!)
Not sure what to tell ya - a better picture with a macro/closeup setting might help? I've had some luck using a paste of saltwater and kalkwasser layered on hydoids to make them go away. I've got about 5 patches of them in my tank, but they don't really cause any issues. But they can be a problem if they get out of control.