Please ID hitchhiker on my LR

Hello. So today I was looking at my hermit crab on a rock and noticed a small hitchhiker on my LR. I tried taking the best pic possible in the hopes that someone can identify it and let me know if it is a pest or to just keep it there.

if something goes near it, it will pretty much disappear into the rock and then come back out and it does it real quickly. The pic is showing it from above and I will try to get a better pic if everyone is having a hard time figuring out what it is.

Thanks

 

Mini feather duster, completely good for the tank.

 

Thanks for the quick reply and the ID. ;D