Good or bad hitchhiker?

I just added 5 thin striped hermits to my tank and i noticed that at least one of them has these 2 little white pyramid looking things on its shell. They have what looks like a bunch of tiny little fingers that reach out and then pull back in, and they do this very fast and quite often. Help Please? Thanks

 

they are tiny starfish that came on a piece of live rock. i have them too adn have just decided to leave then be and they have done no damage to my anemone and sponges. congrats

 

starfish that dont move...with fingers?

 

He is talking about asterina stars that can also become a pest because of their reproduction rates. That also does not sound like what you have. You will probably need to take a pic on this one. To me from the discription, sounds like a harmless feather duster. White tube like body, usually reddish brown "fingers"? by chance?
Many small featherdusters show up on live rock and other things. Large feather dusters are quite pretty as well.

 

yes the body is white but its not really "tube like", larger at the bottom than the top. Its almost pyramid shaped. it does look somewhat like a feather duster, this thing is clearly not mobile. ive seen a pic on here somewhere of one but i cant find it. my camera can focus on it clearly enough to take a pic.

 

Could it be a sycon sponge? Really could do with a pic...

 

I suspect your crabs have barnacles growing on them?

live barnacles on a crab shell on Flickr - Photo Sharing!

 

that pic of the barnacles looks exactly like them. are they bad?

 

i read that they filter plankton and detritus...so good im guessing?

 

yes if it is those in the picture then thay are great and if you want more you can usually find them on oyster shells from the markets, the shells are also good as they will add to the calcium in your tank