Can anyone identify these?!
They are colonial hydroids. Some people swear they will take over, and I can say I currently have a boatload in my tank and wish I'd have known what they were and worked on them early on. They don't seem to hurt anything that I've observed, but they are an eyesore.
Kalkwasser paste helps to control them, but you'll likely need multiple applications.
It is a violation of federal law to use this product in a manner inconsistent with its labelling.