IMO (based on pictures/description) they are one of the thousands species of colonial hydroids. IMO they are a huge neusance that will quickly take over every inch of your tank. Not all, but many can sting corals.
My research/experience with them:
In my 57 I started with 3 small patches about the size of a dime on my liverock. I had 3 different types of hydroids. I had 60+ lbs of liverock, so I didnt think much of them because they were in such a tiny area.
2 of the species grew fairly slow. After 3 months of growth, the dime sized area became a quarter sized area. I decided I didn't want them growing anymore, so I took a knife and scratched the area really good for 5 minutes. Never saw them again.
The 3rd species i had was another story.
Common name is pompom hydroids because, well, they look a little like pompoms.
Here is a picture:
These hydroids entangle into the rockwork like tree roots and new heads were always popping out of the roots. I went from about 20 to thousands in only a few months. In my experience, nothing eats them. I tried peppermint shrimp, keyhole limpets, bhergia nudi's, and a copperband. Pretty much everything that may eat it, and no luck. I tried kalk and glueing over, but it never killed the roots in the rockwork. In a few days/hours they would just reappear. I took my rocks out and took a mini torch to them. Burned the whole areas good, killed off some coraline and sponges though. Looked good for 2 weeks, but then they just came back in full force like I never did anything. I tried manual removal in tank, but if one floated away it just started another colony elsewhere. The hardest part was the 'roots' which went inside the liverock and were next to impossible to completely exterminate.
I eventually took off all my corals and threw out all of my rock. I had to drain my tank and clean it because a few hydroids were on the overflow box and it only takes a few to become thousands in notime.
Sorry, but if it were me I would save the time and hassle and throw out anything that may have hydroids on them. I was much happier and more relieved when I just threw all the rock away and started over. Maybe someone else has had better experiences, but for me any many others online, it was better to just toss everything. You have to make sure you get every single one though...Its tough because some may be on the sand, glass, overflow, coral bottoms, powerheads, ect.
They are probably so infused and webbed in your liverock that complete eradication is probably very hard. Yours look a little different than mine so I am not sure. There are thousands of species, and we know relatively little about them.
