What are these.

[Gooners]

since setting up this tank 12 month ago, I noticed these green looking flower heads.
I cant find any info on line about them.
So i was wondering
1, what they are?
2, are they likely to cause any harm?

 

[WV Reefer]

since setting up this tank 12 month ago, I noticed these green looking flower heads.
I cant find any info on line about them.
So i was wondering
1, what they are?
2, are they likely to cause any harm?

Hard to tell for sure from that pic but they look like hydroids to me.