Hello and thank you for your inquiry.

Luckily (knocking on wood) this is on pest I have not had to deal with in my reef aquariums to date so I don't have a lot of experience with this.  So I went to the Marine Depot Forums (in particular Dr. Ron Shimek's forum) and searched for some help.  I found this thread about hydroids that I hope will help you out:

http://forum.marinedepot.com/Topic99178-11-1.aspx?Highlight=hydroid

Hopefully that helps with your issue.  You may also want to snap a good picture of them and then you can post on the MD forum as well if the above thread doesn't help with your issue. 

If there is anything else we can do for you please let us know.

Keith M.

Marine Depot Customer Service