Are These Hydroids?

[Alaeriel]

Just did a water change yesterday and now I see these guys. I've counted four, but I have no idea where they came from. Snails, maybe? The tank has only been set up since August and we started with dry rock and "live" bag sand. Two are stuck to the glass and the other two are tumbling around in the current. If they are hydroids, any indication of which species? So far we just have some snails and hermit crabs in the DT but were hoping to set the firefish out of quarantine today. Should we hold off? Some sources say hydroids are terrible, some say neutral, some say they disappear on their own. Thanks for the help! Let me know if there's any other information that could be helpful!

 

[BranchingHammer]

I had these in my frag system for a little while when I first set it up. They disappeared after a little bit though.

 

[stanleo]

I think I remember they are a type of jelly fish medusa. Harmless and disappear as the tank matures.

 

[Andreas' Reef]

Some sort of jellyfish. They disappear after a few months.