I think this is just a type of snail. The worst it can do is irratate your corals . Pick it off of the rock if you want it gone.
This is a type of pest anenome. You might want to kill it before it gets a chance to spread.
This one looks like a hydroid colony to me and should be removed IMO. They are not good to have, but I'm not 100% positive that this is one.
Hope this helps.
