What is this?

[Spec V]

Came from out of the blue. I haven't added anything to my tank in about a year. They are tiny and the cluster is about half inch wide.

 

[Reefing Madness]

Colonial Hydroids

 

[Tahoe61]

^Yup.

 

[Spec V]

Now how do I get rid of them

 

[Tahoe61]

If that is a small patch cover it with a piece of epoxy/sw, or remove the offending rock and scrub it off, or try kalk paste.