what is this growing on blue leg hermit?

[SmokinReefer845]

i have about 10 of these guys in my QT. he is the only one with this on his shell and they have been here for months he acts normal. any ideas? should i keep him out of main display

 

[shwareefer]

I'm going to say hydroids.

http://www.wetwebmedia.com/hydrozoanfaq2.htm