Hydroids or.....?

[amcarrig]

Any ideas? These things are stinging the heck out of my monti caps and I'm not sure if they're hydroids or something else so I don't know how to proceed with eradication. These are currently in the overflow box but they are also growing on some of my rock. Thanks in advance!

 

[graveyardworm]

They are hydoids, I have very similar ones which so far havent posed a problem. They are tough buggers to eradicate, and I cant help you in that department.